How do I once ive found an address and have the private key do I put it into Multibit.
Google: multibit import private keyhttps://multibit.org/help_importASingleKey.html
Also is it possible for someone to use the same software and find the same private key. and then spend my funds?
Possible: Yes. Just highly unlikely. Vanity addresses aren't 'special' in technical terms. If they could find a collision for your vanity address, then they could find one for any address.I have my wallet encrypted is that enough?
That's a general wallet security question - the addresses (or private keys) stored within are unrelated to that 
