I was considering it for a 4x8 build.

I'm not sure you would lose that much strength, plenty of people make torsion boxes using only glue for joining for example. The vacuum process is really to squeeze the different layers of cloth together around a complex shape, when you are dealing with a ready made flat sheet you can achieve the squeezing the old fashioned way with weight or clamps. If you were to try to make the gantry in one piece then yeah you would need a vacuum to ensure good contact in all areas.