If it was me, I might either lighten up the blue background like Paula suggested, or go with white- then your logo will stand out more. Or, you could make a darker logo (also easy in Lumapix) to go with this STD.
I don't remember for sure, but I think you can upload a completed project .jpg to vistaprint, and therefore use your own fonts. I think there is a size limit to the upload though, so make sure your finished project is small enough. Does that make sense?
Another layout idea (to keep it simpler, and make your logo stand out more) would be to just to 2 large pics (I like the first two from the left the best, just my opinion) with your logo in the middle, and text underneath it. I'm thinking that by the time this prints out as a postcard the pics are going to be pretty small, and it might be better to emphasize a couple of them and see them well.
But, if you leave it the way it is, it's already good. If you don't make any other changes, I would just make the picture on the upper right align with the one underneath it, and do the same with the 2 middle photos...it will look more balanced that way. Just a couple suggestions!