Yeh pumps are only on or off, you can't control the vacuum with the pump. You need a vacuum regulator. This can be as fancy as dedicated composites vacuum regulator, or as cheap and simple as a fish tank air valve. It is essentially just introducing a leak in a controlled manner, so you leak away half of your vacuum or whatever your target is.
Or, if you know what you're doing and you have a perfectly sealed bag, you can just pull roughly half vacuum and shut off the pump. But this is very reliant on your bag, if you have any leaks at all then all bets are off.
To confuse you even more: The absolute best method is actually quite similar to pre-preg, where you want to work out the viscosity of the resin system in relation to its cure cycle. If you can hold the vacuum level at quite a low vacuum while it is at its lowest viscosity, and then ramp up the vacuum as the resin viscosity goes up, then you can time it so that you reach a full vacuum just before the resin cures. By doing that, you minimise how much the air can expand with the vacuum but you still get full compaction. Very tricky to pull off.