Virtualbox seems even more complicated since virtualbox is in jessie but virtualbox-fuse is not. And virtualbox-fuse depends on virtualbox.
There is no correct solution for non-existing packages, but only more or less elegant solutions.
virtualbox-fuse has been dropped, since it is incompatible with the current virtualbox version (see the changelog of Debian's virtualbox package).
So if you need it, you need to forward-port the whole virtualbox packages from wheezy (which includes virtualbox-fuse). Forward-porting works like back-porting.
As jessie is similar to unstable, you can try to install the current gajim package from unstable without updating other packages.
But this gajim version has a grave bug which could be the reason why it has been temporarily removed from jessie / did not migrate to it. As soon as the bug is fixed, it will migrate automatically to jessie. Helping the maintainer fixing the bug can speed this up.
So forward-port it from wheezy can be also a good solution.
If offered, using packages from non-official repositories / repositories from related distribution can also be a solution. Compiling software by yourself should work, but is in general not easy.