a couple of factors to consider are how long your chamber takes to get to full vacuum and also pot life. I would suggest a good 5 minutes at full vacuum should be plenty to remove any air that is going to come out.  This is subject to having available pot life for the resin which with our water clear PU is ok at 12-17 minutes.

Is the silicone mould supported in a box? Supporting it may help prevent it being shrunk in the vacuum process. Alternatively, if there are no undercuts or features that could cause mechanical locking then moving over to a rigid mould may be a better option. However, the disadvantage is the need to use release agents and to think about how you will prise/release the part out without damage.

I' have not heard of vacuum bags being used in resin casting.  In theory you could modify an infusion type process to suck resin into a mould. However the design would need to be carefully though out so that the resin has paths to flow and also that the vacuum bag doesn't just press tightly against the mould surface leaving no where for the resin to go.  A variation could be on RTM light with a closed mould and the resin sucked into the void in the closed mould that way instead.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant