better to marinate in vacuum?
Marination, contrary to popular wisdom, is a surface treatment.  Meats are simply not that porous; the only molecule small enough to penetrate more than a millimeter or two beneath the surface is salt, which diffuses at a rate of about 2 cm per 24 hours.

So I would choose the route of convenience:  do you want the marinade in the bag as you are going through the cooking, perhaps as the basis of the sauce?  In that case, add it and seal.

Otherwise, complete the marination and then seal.