Jürg Billeter created Vala for the sole purpose of building GTK / GNOME applications and the two projects are tightly coupled.
It's not to say that Rust won't have GTK bindings in the future, but it's not a high priority for either Mozilla or GNOME devs and unlikely to displace Vala.
I would have thought that considering they are building a browser that was originally written in C-GTK, the first order of business would be to build GTK bindings.
I would say Vala and Rust do have fairly similar goals in general.
P.S. I know right now Servo != Firefox, but I suppose it will get there eventually.
Vala is only coupled with GLib for the object system and even that can be replaced by using another profile: https://wiki.gnome.org/Projects/Vala/Tutorial#Profiles