Recently, i have had real trouble coming up with vocal melodies. I've only just started writing songs (Only music, no lyrics, and only guitar), and everytime i try and think of a vocal melody, i hit a wall. I can't seem to come up with anything good.

Any ideas on how to come up with a half decent vocal melody?
learn your scales, basic major and minor will do you fine at first and then go over the scale of whatever the key is of the song and improvise on guitar till you get something that works over the given chords.
Thats how ive been doing it for ages, works fine for me
Just sing random stuff over music...go on for as long as you like. Record it if you can. Play it back and look for things in there that sound good. Just let go and sing whatever comes to mind, even if it's just gibberish.
Write the vocals as a guitar, or even better, derive a melody from your solo (if you have one), e.g. Smells Like Teen Spirit
Guitars & Gear:
Parker Nitefly M
Sumer Metal Driver
Ibanez RGD2120Z
Two Notes Torpedo CAB
Another way of finding your melody would be to really have something to say... I mean really want to share something with people through your musical piece. Personally I find that melody comes very easy when I really get into the tune I'm working on, and then let the first word that describes the emotion come out.
"Life is just a ride... just a ride" - Bill Hicks
Mostly I start a song with an idea for a vocal & guitar hook

Humans are naturally wired to pay close attention to other human voices, so I tend to think that often the vocals have to be pretty central to a song, rather than something you add "over the top" once the rest of the song is worked out

But sometimes to work on verse parts.. I just sing nonsense syllables while playing a riff

Just a few thoughts, feel welcome to take them or leave them. there are no absolute rules.