First of all, got everything replaced on the cooling system but my friend bought a 13 lb radiator cap instead of a 16 lb. The coolant is not being sucked back into the system from the reservoir but is blowing it out of the side hose on it. Is that due to the 13 lb cap? Second question, The car is sputtering the first few times I start it up every time then dies probably twice then it'll run normal and sound normal while idling. I found a vacuum line not connected when fixing leaks in the water pump and housing, Would the disconnected vacuum line be the reason for it having such trouble on starting up?

 

If the car is spewing coolant out the reservoir, it could be:
1. Air in the system
2. Bad coolant seals (blown engine)

Use a coolant system tester, or first run car with front slightly elevated from rear. Remove rad cap. Run heater full blast. let idle for 15min. That will remove all air. If while doing this coolant is constantly spewing out, you have major issues.

Vacuum lines will not cause coolant system issues.