I think the title explains it already...
As @aperson said, Geany is a very good text editor - It is lightweight with lots of features. It also supports vala (you need to install
valac first though for full vala features). A lot of its features are IDE like eg. you can build/run with 1 click.
To install, run
sudo apt-get install valac geany or search for 'valac' and 'geany' in Ubuntu Software Centre.
There is new project called Valama, you can check:
It uses gtksourceview, so editor experience is similar to gedit, but it's still in early development phase.
UPDATE: It is getting better every day, there is active development on going.
There are two plugins for Gedit that provide Vala support. Valencia and VTG both add autocompletion, symbol browsing and basic project management through makefiles
Valencia is the easier of the two to setup because VTG depends on gtksourcecompletion, but VTG has made several recent releases.
Anjuta supports vala since ver. 2.31.3 and there's a nice plugin for gedit. http://redmine.yorba.org/projects/valencia/wiki
I can't recommend an IDE specifically, but I can recommend Geany as a great text editor.
Val(a)IDE seems to be the only IDE with Vala support, so if you want an IDE that is properly the way to go. Personally I use Vim for my coding needs, I think it makes good sense to use a powerfull editor instead of a single purpose IDE.
Instead of knowing 20% of the commands (keyboard shortcuts) in five IDE's I can get to know 99% of the commands in one editor.
Surprised no one has mentioned Gnome Builder.
Upon opening the IDE, you can see the kinds of projects it supports (which also includes ones written in Vala):