I've had the fortune of watching my cousin do some glass and carbon fiber layups/vacuum bagging. He/they use PVA (Poly Vinyl Acetate) material which can be shaped easily to fit the mould and the edges can be layered/sealed together to actually make a true bag...

The demo my cousin showed was how UN-uniform about 5 layers of cloth are when worked by hand versus the same amount put under vacuum... The "bagged" part was considerably thinner and VERY uniform in thickness in comparison...

Cool video Uncle...