Scenario
I had some players fighting off a vampire's attack. The vampire was a sorcerer and rather than dying to the player characters, he casted Gust of Wind and used his Gaseous Form ability to propel himself pretty far away and in a hurry.
One player started arguing that even in gaseous form he is a still a creature larger than tiny and shouldn't be affected. I told him that it says right there in the description that he assumes gaseous form as the spell, which is affected by wind. Rather than continue to argue with him, I said the vampire casted Dimension Door instead and did what he wanted to anyway.
Party Make-up
8th level Human Paladin of Lawgivers; focusing in smite attacks.
8th level Human Swashbuckler of Van Richten Society; focusing in melee and flanking.
8th level Human Rogue of Van Richten Society; focusing on skills.
8th level Human Rogue of Van Richten Society; focusing on sneak attacks.
8th level Human Cleric Lawgivers; focusing on buffs, debuffs, and healing.
Campaign Details
Party agreed to Assault Castle Ravenloft. However, I designed the campaign for them to have a protagonist version of Lord Strahd that makes several appearances and harrasses the party throughout the campaign, rather than simply being a monster awaiting in a lair.
Also, beefed up the entire campaign so the characters would finish around level 18, and leveling up Strahd to be a level 20 character. He is going to be two levels higher than the party at all times. Also, I made him a sorcerer rather than a necromancer, simply to reduce MAD.
Vampire's Tactics
After trying to Dominate someone, and failing, the party wised up to him. He used a scroll of Haste, as he had won initiative.
Several Sleeps, Charms, and other successful will saves later, the vampire decided that leaving was better than trying to fight it out was.
He assumed gaseous form. He had still spell, silent spell, and eschew materials. He then cast Gust of Wind to propel himself 60 feet away before moving away in gaseous form. He had applied Grease on the ground so the paladin couldn't simply charge attack him after moving away.
Gaseous form was preferrable since the party still had ranged weapons and a severe lack of attacks that could harm his gaseous form. But due to the potential debate (which I hate sometimes) I said, ok, he used Dimension Door and is now 400+ feet away, and instead of using Gaseous Form he turned into a bat and would begin flying away into the air.
Yes the attacks occured at night, not near running water, and not within a building he didn't have permission to be in.
Question
Now… dealing with argumentative players aside, am I correct that the vampire should be blown away in Gaseous Form by Gust of Wind, or would be player be correct that he is indeed still a creature of larger size and wouldn't be affected?
References
Gust of Wind states:
In addition to the effects noted, a gust of wind can do anything that a sudden blast of wind would be expected to do. It can create a stinging spray of sand or dust, fan a large fire, overturn delicate awnings or hangings, heel over a small boat, and blow gases or vapors to the edge of its range.
Vampire's Gaseous Form states:
As a standard action, a vampire can assume gaseous form at will as the spell (caster level 5th), but it can remain gaseous indefinitely and has a fly speed of 20 feet with perfect maneuverability.
Gaseous Form states:
The creature is subject to the effects of wind, and it can’t enter water or other liquid.
Best Answer
Yes, you can.
As stated in the spell Gaseous Form:
Emphasis Mine, size doesn't matter in a gaseous form, as you will be subject to the effects of wind as any other gas would be
With Rules as Written, you can definitely cast Gust of Wind as a standard action, then using your move action to move into the gust, to carry you along.
If I was GM though, I would rule against this use, as the spell to me seems intended that a single blast of wind comes from the caster, moving away. The actual ability of the caster to get caught in the gust seems very ambiguous. It's your game though, and if you feel that the Vampire would be best served as a storyline villain, and not just a once off, that's something to discuss with the table. Just make sure you don't have anyone ready to snipe him off after the 60 ft. that he moves.