Vac pump has to run till the resign has gone off, about 6>8 hrs with epoxy.
The ideal is to use peel ply over the wet out laminate, then bleed cloth over the peel ply, then shade cloth or similar over the bleed cloth.
Leaving the pump running during the cure time ensures that all excess resign is drawn out of the laminate into the bleed cloth ( felt or similar ) and ensures that the layers of cloth in the laminate are bagged down without pockets of air or resign between them. This is vital for strength and weight.
The shade cloth helps the vac bag to come down with even pressure over the whole area cos it allows the air to be drawn out of the bag and not trapped in different areas . The shade cloth only needs to be a strip running the length of the bag, with the vac hose sitting on top off it.
If you try what you were asking ie bag down then lock off and remove pump the bag will lose pressure and you will end up with voids in the laminate that will delaminate quickly in use.
The best type of home vac pump is an old fridge compressor. They are cheap and can be converted to suck easily and can be left running for extended times with out any problems.
Hope this all helps