On thick section rubber moulded parts, you need time for the gases ( given off during heating/curing) to build as the rubber takes heat from the hot mould. Don`t forget rubber is a good insulator so even if you pre-warm the rubber, the mould temperature will still take time to penetrate the whole section of the rubber slug as it is only in contact with maybe 3 hot mould faces maximum when you first load it into the mould.
Bump too early and you won`t have any effect other than allowing air into the mould. Bump too late and the part will have begun to cure so will distort as the mould closes again. Check the rheometer cure trace and bump as late as you can without causing part distortion and only have a small bump opening gap of up to 0.5mm max.
This way the volatile gases in the rubber will have started increasing the pressure in the cavity so when the mould opens slightly, the gases that are under pressure will rush out to the lowest pressure area, which is outside of the mould cavity not inside it. The small gap will increase the velocity of the gases out of the mould, like putting your finger partly over the end of a hose pipe makes the water velocty incease, so don`t have your bump timing set for too long. 2 or 3 seconds should be easily enough.
Bumps are still needed even if your press has vacuum, as the vacuum only tends to intialy remove air from the gaps around the uncompressed slug and the mould cavity contours. Once the slug is compressed and formed by the mould being fully closed, the vacuum is effectively sealed off from the inner part of the moulding by the moulding itself. Hope this is of some use.