That dependes on how do you interpret Weigthless and the context. I can think of two interpretations:
Weigthless means the vampire has no weigth or has a nelligible one. That that it behaves like a helium-filled ballon.
Weigthless works like the aD&D spell reverse gravity
Now we can devise two posible contexts:
Conflict happens in a open space:
In case 1, he is subject to the whims of wind (remember, he now is
like a ballon). He needs to use some method (has a fan at hand? or a
hook?) to propel itself to the ground or to any place or object he
can use to anchor himself until the effect ends.
In case 2, he will be really screwed, since he will be freefalling
upwards for 5 rounds and then fall to the ground again. Unless he can
fly, has a parachute, or can use somewhat to soften his landing, he
is for a painful experience.
Conflict happens in a enclosed space:
In case 1, with no risk of being taken to the sky by a gust of wind,
he may try to move the same way astronauts do in a space station: Using
his arms and legs to rebound off walls and ceilings. The vampire of
course lacks the appropiate training to do so, as pointed by
SevenSidedDie, but since we are talking about a supernatural being,
the strongest villain in the encounter on top of that, and that we
are talking about using magic to temporarily suspend the laws of
physics to begin with, we are going by the premise that he would fare
better than ordinary humans in the same situation, and that, even
while being severely impaired due to having to deal with effects of
weightlessness, he is, however, far from helpless. Discipol suggested
treating the condition as a snare. The idea is good, but since he is
not actually attached to a place, tied or paralized, I also add one
different approach: Instead of restricting his movement, you make him
unable to control them, forcing him drift through the scene
erratically as he receive impacts and try to attack and dodge.
Imagine , for example, what would happen to him if he tries to fire a
gun without putting his back against a wall or similar.
In case 2, now we have a vampire walking by the ceiling. He act and
moves normally, save the fact that he treats floors like ceilings and
ceilings like floors.
If your DM judges it works like case 1, then the spell is fine, as it inflicts the targets with a impairment severe enough to disable weak enemies or cause problems to the strong ones, but without completely disabling them.
If your DM let it works as the case 2, he may consider to forbid the spell where there is no ceiling, reduce the duration, or otherwise nerf it, so that a imparing spell do not turns into into a unsuspectedly strong damaging spell.