Depends what paint you're shooting. If you are shooting 4 or 5 star, you will need the spring as it will just break paint. If you are shooting 3 star or lower and the paint is a lot less brittle then you should be ok without it. There is a rumour going round that removing the spring gives your Vanquish a softer shot (this is rubbish).
If the marker comes with a spring in the bolt. It is probably there for a reason. I would get hold of a spring if I were you. Whichever paintball retailer you normally use will probably sell you one for a couple of quid.