If I am attempting to prepare for preformance, I usually don't seriously try to sing until I have the guitar part down pat.  I find that vocal control takes more of my concentration than playing the guitar.  If I am struggling to play the guitar, both my guitar playing and my vocal usually sucks... and my singing rarely rises to the level of performance worthy.  I try to get the guitar part so drilled, that I can reliably play it without having to think about what I am doing.   Only then can I begin to really work on singing and playing at the same time.