I'm not entirely sure of how this works, so you may have to explain to me as we go along
Here's the public key I've just generated: 1hueY3mDiCcof3QSeFmBk7VSHnnyXp1hJ
I have made a note of the private key.
After I posted my suggestion I realized that in order to combine the partial private key with your private key you'd need to get the keyconv utility from the vanitygen binaries zip file here,
https://github.com/samr7/vanitygen/downloads (get v0.19 not the older 0.18)
But then if you get that then why not just run the vanitygen program yourself...
To my knowledge there isn't an online way to combine keys yet. So if you still want to do this I'm game but it may not offer much benefit unless on your system vanitygen takes many days to run. In which case this will still save you lots of time.
All things considered, I think I will spend the time getting vanitygen to work - that way I can generate as many as I like