As Clay mentioned, size of the print and the type of light source in use makes a big difference. If you have a good print frame and are printing with a collimated or point light source a vacuum frame may not improve sharpness, or at least not very much. But if you are making large prints with a diffuse light source, a bank of BL fluorescent tubes for example, the switch to a vacuum frame will almost certainly result in a significant improvement in sharpness.
If you have any question about how well your printing frame is working make a few prints with a diffuse light source and a few with the sun, the ultimate point source light, and compare the differenece.
Sandy King