Gaseous State: Vapor can use Smoke Cloud. If she does, modify her defense value by +2 until the beginning of your next turn. Whenever a hindering terrain marker placed by her is removed from the map, you may place her in that square.

When I reposition Vapor after removing one of her Smoke Cloud tokens, could I then use her Poison?
Sure, assuming that the removal of the token and placement of Vapor occurs during the beginning of your turn. Normally, Smoke Cloud and Barrier tokens are removed at the beginning of the turn before your characters can do anything else (including use Poison), so the timing usually works out just fine. However, some game effects, like CR #005 - Speedy's Trick Arrows, that can cause terrain tokens like Smoke Cloud to be removed at other times; if that were to happen, you could still choose to reposition Vapor (one of her tokens has been removed after all), but due to the timing of the removal, you'd have to wait until the beginning of her next turn to use Poison.

If all of Vapor's Smoke Cloud tokens are removed from the map before the beginning of her next turn (pretend I'm playing two CR #005 - Speedy characters), will Vapor lose the +2 modifier to her defense value?
No. The modifier is not contingent on the Smoke Cloud tokens remaining on the map. All that is required is that she used Smoke Cloud; once she has done that, her defense value will be modified until the beginning of her next turn or until she loses the power.