First of all, sorry if this belongs in the songwriting/lyrics section, please refer me there if thats the case, but I just want to know if its a good idea to write a song on guitar, and add a vocal melody according to that song, or write a vocal melody first and write the guitar parts according to that. Which one seems to work better?
Vocal on top of guitar, just cuz.
both are totally fine. lots of people find it easiest to work on both at once, but the real factor is inspiration. if you hear the vocal line in your head, write that first so you have it to refer to when you work out the guitar part. If you jam out an awesome part on guitar but don't have vocals, you can still work on the guitar part as much as you want.