You are allowed to install one Vegas license on multiple computers as long as you own them all and only use it on one at a time. If that doesn't cover your situation then you would have to buy another license. I've never seen any discounted prices. Probably the only people who could answer this authoritatively are SONY's customer support. Give them a call and ask.
You may be able to get a "special" extra license for an additional network rendering node by calling Sony (I saw posts about this previously) but that is not the same as a "real" full license for Vegas.