bike computer mounting

I have the same problem with my Cateye Mity3. It doesn't fit the fork snuggly. I fixed it with some black electrical tape. You can hardly see the tape against the fork. Works fine.
You'll have to check the Vello site to see if they have a replacement bracket for you, or ask the dudes @ your LBS.

Ie. you just used electrical tape to tape it straight onto the fork/paint?

My fork is shaped like most carbon forks, flat on one side and rounded on the other. The bracket for the speed sensor is made for a standard fork, rounded all around. What happens is that the bracket only catches on the one side of my fork when fitted normally. So what I did was to attach the bracket with plastic ties as per normal, but on the flat side of the fork where the ties don't catch, I used a piece of eraser between the fork and the ties. The nice thing about using an eraser is that it won't damage the fork and that, when compressed, it tries to "kick back" to it's original shape. You can probably use any piece of rubber, it will give the same results. I then support the whole thing by wrapping some electrical tape around the bracket. This prevents the bracket from moving around.

Hope this makes sense?