D&D 5e just isn't this simulationist
The question can't be answered by the published D&D rules, as the game is not meant to simulate such interactions in any way 'realistically'. The intent of Gaseous Form is to give various advantages of movement and protection to the user, and though there are limitations it is not meant to be accidentally deadly. Weather in D&D is often used to provide atmosphere, and what small space is given to mechanical details in the DMG does not cover this kind of hypothetical situation.
Gaseous Form
The spell description gives all the benefits and limitations expressed by the rules, and it says nothing about rain. Though the description says:
The target can pass through small holes, narrow openings, and even mere cracks , though it treats liquids as solid surfaces,
the intention of this is to allow the beneficiary to move like a gas, giving some benefits, but be stopped by solid bodies of liquids, like the surfaces of rivers, lakes, seas etc.
Rain in D&D
As a DM, I use weather mainly to provide atmosphere, and this approach is supported by some advice to DMs in the Starter Set:
Describe the party's overland travels as vividly as you like, but keep the story moving. "You walk for several miles and encounter nothing of interest" is far less evocative and memorable than, "A light rain dampens the rolling plains as you travel north. Around midday, you break for lunch under a lonely tree. There, the rogue finds a small rock that looks like a grinning face, but otherwise you see nothing out of the ordinary." (Lost Mine of Phandelver, 27)
Of course, the fact that the DM has described it is lightly raining can, at the DM's discretion have some mechanical implications (e.g. walls are slippery and so the DC is higher to climb them). But the only generally-applicable mechanical information about rain that I am aware of is in the DMG, p. 110:
HEAVY PRECIPITATION: Everything within an area of heavy rain or heavy snowfall is lightly obscured, and creatures in the area have disadvantage on Wisdom (Perception) checks that rely on sight. Heavy rain also extinguishes open flames and imposes disadvantage on Wisdom (Perception) checks that rely on hearing.
Note that this is only describing an extreme weather condition (heavy rain, not any rain), and in any case no mention is made of magical effects like gaseous form.
Yes, you can.
As stated in the spell Gaseous Form:
The creature is subject to the effects of wind, and it can’t enter water or other liquid.
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
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.
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.
Best Answer
Gaseous form makes you resistant to nonmagical damage and keeps you from falling
The Potion of Gaseous Form has the following description:
And the gaseous form spell description states:
It severely limits your ability to interact with your environment while in that gaseous form, but correspondingly, it also protects you from some of the negative effects of having a solid form. You have resistance to all nonmagical damage (e.g. damage from being hit by debris), and you also have advantage on Strength and Dexterity saves (e.g. to avoid being hit or pushed by debris).
So, depending on how your DM would have ruled the fall and how it might have damaged had you stayed and not used the Potion of Gaseous Form, the potion might have protected you from the worst effects of the collapse if you had stayed and used it.