It has been managed by tarnyko to compile Vala sources under Android and be able to use Cairo. GTK+ has not to been ported. I'm quoting the interesting part of message verbatim below:
I managed to compile a Vala source directly to Android binary yesterday. And I think it might be of some interest to some people here.
For the record, I've written a tutorial here: http://www.tarnyko.net/en/?q=node/25
It's still very limited (only access to core Vala + GLib/Gobject methods) but it could easily be extended by porting libraries having Vala bindings ; although I know it's not easy stuff.
For the graphical part, GTK+ doesn't work yet but I know Cairo has been ported (http://code.google.com/p/cairo4android/) -and Cairo definitely has Vala bindings.
More information at the Vala programming language homepage.