Sure. They'd see the gas.
Yar.
On the other hand, Gaseous Form is a transmutation, regardless of how long you can keep it up.
...and sees the true form of polymorphed, changed, or transmuted things.
So a Vampire in Gaseous form will be seen as a Vampire by someone looking at with with True Seeing. However, this doesn't remove any of the other effects of Gaseous Form. It just lets you see it for what it really is.
Also: A vampire's Gaseous Form ability is still a supernatural ability that they have to activate. Even if they can maintain it indefinitely, it is still not their natural form.
Heck, Pixies have the Invisibility ability, with the special clause that it is actually constant, and they must consciously make an effort (albeit very little effort: Free Action) to suppress it! I'd still rule that True Seeing would let you see the Pixie. They would also become visible in an Anti-magic field, as it is a (su) ability. Same with a Vampire's Gaseous form being negated in an anti-magic field, as it to is (su).
The better question would be this: What would someone with True Seeing see when they looked at an Invisible Stalker? They are the one creature that is "naturally invisible".
~P
Sure. They'd see the gas.
I disagree.
You confer on the subject the ability to see all things as they actually are. The subject sees through normal and magical darkness, notices secret doors hidden by magic, sees the exact locations of creatures or objects under blur or displacement effects, sees invisible creatures or objects normally, sees through illusions, and sees the true form of polymorphed, changed, or transmuted things. Further, the subject can focus its vision to see into the Ethereal Plane (but not into extra-dimensional spaces). The range of true seeing conferred is 120 feet.
They'd see the gas and the vampire.
I stand corrected.