The aerodynamics don't really favor you with a fixed blade broadhead and straight vanes.
As we all know, fixed blade broadheads have the tendancy to plane. If you use straight vanes then your arrow will not spin in flight and allow any small misalignment of the broadhead to continue to plane in the same direction and push the arrow off course. With a helical or offset fletch, the arrow spins, continuously changing the planing force of the broadhead over the 360 degree rotation. This in effect averages out the effect of the planing and keeps you on course. Theoretically the faster the arrow spins, the better accuracy you will have as the instantaneous deviation from straight flight will be smaller (arrow will always be closer to being dead on target), but this leads to other issues in downrange energy and penetration, overall arrow balance, etc...

That was the long answer. The short answer is no, don't go with straight vanes on a fixed blade broadhead, your accuracy will suffer.