Note: This answer is for dnd-3.5e exclusively. Other answers may provide pathfinder specific advice.
The Monster Manual on Slaying a Vampire says, “Exposing any vampire to direct sunlight disorients it: It can take only a single move action or attack action and is destroyed utterly in the next round if it cannot escape” (253). Libris Mortis expands on this description, that text on Sunlight Vulnerability saying that this means
…direct exposure to the light of the sun. Reflected sunlight, whether via a mirror or the moon itself, has no effect on an undead creature vulnerable to sunlight. Cloud cover or similar interference does not protect the undead creature unless it is thick enough to provide concealment to the creature. For example, a vampire within a fog cloud spell would not adversely be affected by sunlight. Even thick clothing, as long as it covers the body completely, can protect an undead creature from the dangers of sunlight. (140)
That text’s Table 7–3: Undead and Sunlight (ibid.) says that sunlight exposure leaves a vampire “disoriented, destroyed” then provides a note saying, “One round after exposure to sunlight begins, a vampire that remains exposed is destroyed utterly.” However, the Monster Manual remains the primary source for a vampire, and that means—despite Libris Mortis—during that 1 round the vampire still exists after it’s exposed to sunlight—when the vampire’s disoriented—a vampire is still limited to only either a move action or a standard attack. This limitation on the vampire’s actions makes it incredibly difficult for a vampire to activate effects that can rescue a vampire from its impending destruction.
For example, even if a DM were to allow the effect to function in such a way—and most won’t, making that an incredibly big if—, blotting out that pesky sun via the effect of the Iron Heart maneuver iron heart surge (special) (Tome of Battle 68) is impossible for the typical disoriented vampire as such a vampire must take a standard action to employ the maneuver. Further, for example,—assuming the rules for doing so from the Rules Compendium are in use (85)—activating a wand of quickened no light (trans) (Book of Vile Darkness 100) (0-level spell at caster level 1 modified to a 4th-level slot by a level 7 caster) (420 gp/charge) is also impossible, a disoriented vampire being unable to take even free actions, much less a swift or immediate action.
With all this in mind, for a vampire on a budget, listed below are what I think are the most cost effective ways for a dnd-3.5e vampire to avoid destruction from exposure to direct sunlight.
If the DM’s hewing closely to the rules, an explorer’s outfit (PH 129, 131) (10 gp; 8 lbs.) probably won’t be enough to cover the vampire’s body completely, so a cold weather outfit (PH 129, 131) (8 gp; 7 lbs.) likely won’t be enough either. A vampire in such a campaign needs an outfit that says it covers the wearer from head to toe, face included. The black bodysuit (Arms and Equipment Guide 29, 30) (30 gp; 1 lb.) may be sufficient, although to reap its other benefits also the vampire must be lightly equipped. However, the most foolproof method of covering everything according to the rules is probably the hydration suit (Sandstorm 99, 101) (1,000 gp; 10 lbs.), essentially a Dune-style stillsuit with goggles and everything (from the novels not the films). However, a more generous DM could rule an explorer’s outfit sufficient, maybe supplemented with piecemeal clothing from the Arms and Equipment Guide‘s Table 2–2: Clothing (29).
However, for a vampire, the terrifying thing about relying on this method to protect it from a sunlit environment is that clothing—like other worn items besides armor—is vulnerable to sunder attempts… and sundering a worn object is particularly easy: not only is doing so usually easier than hitting the vampire, but also clothing typically doesn’t have much in the way of hardness or hp. Seriously, once a dnd-3.5e vampire that’s out in the open on a sunny day is recognized as a vampire, the good folks of the town should do their best to rip its clothes off.
…Then take the feat Endure Sunlight…
The monstrous feat Endure Sunlight (LM 26) has as its prerequisite a vulnerability or weakness to sunlight—like that of the typical vampire—and has as the following benefit :
You can resist all dangerous effects of sunlight for a number of rounds equal to 1 + your Charisma modifier (minimum 1 round). After this time, if you are still exposed to sunlight, you take the normal effects as appropriate for your kind.
This gives any vampire at least 1 round to do something other than move or attack, like activate a wand of obscuring mist (conj) (PH 258) (1st-level spell at caster level 1) (15 gp/charge) (just as effective as the 2nd-level Sor/Wiz spell fog cloud (conj) (PH 232) for the purpose of saving a vampire from destruction), that aforementioned wand of quickened no light (without the quickening works, too!), or even—if the DM says it’ll make a difference—the martial maneuver iron heart surge. However, if the vampire wants true reliability in avoiding its destruction, it should probably have on hand a very particular potion (see below).
…Then chug potions of cloak of dark power
A potion of cloak of dark power (abjur) (Spell Compendium 48) (1st-level spell at caster level 1) (50 gp; 0.1 lbs.) should be available in all but the smallest town, despite the spell itself being available only as the 1st-level spell of the domain Drow (273). The spell’s effect creates around the subject a dusky haze that “does not interfere with vision, but the subject and anything it wears or carries is protected from the effects of full sunlight, even under the open, daytime sky of the surface world.” The spell’s 1 min./level duration should be sufficient for a vampire to get to safety after the vampire’s clothes have been ripped off and its rounds of sunlight endurance nearly exhausted.
If the DM rules the potion isn’t available—a strong possibility given the rarity of casters who can trigger the spell so as to create a potion of it—, a vampire can take a full-round action (that somehow doesn’t provoke attacks of opportunity) to spread on itself the alchemical substance liquid night (LM 74) (150 gp; 0.5 lbs.). Doing so protects the vampire from sunlight for 1 hour. The cost—both in actions and gp—is higher than for the potion, but a vampire lacking access to the domain Drow may be able to itself manufacture liquid night instead of needing to purchase potions of cloak of dark power from increasingly suspicious vendors. However, both the potion and liquid night have achingly obvious effects—the haze of the potion‘s effect and the coating of liquid night being a “dark, sticky fluid” that “has a distinct musky odor of moonflower”—, making the presence of either effect a (ahem) dead giveaway that something’s not right with the affected creature.