I inspected the dependencies of the virtualgl* packages in the PPA.
bumblebee recommends virtualgl | primus,
virtualgl recommends virtualgl-libs-ia32 (i386) (also depends on virtualgl-libs),
virtualgl-libs-ia32 (i386) depends on virtualgl-libs (i386 too, I think),
virtualgl-libs depends on libgl1-mesa-glx | libgl1,
virtualgl also depends on libgl1-mesa-glx | libgl1 and several other dependencies of virtualgl (redundant dependencies maybe?).
I suppose this is why installing bumblebee-nvidia from the PPA ends up also installing libgl1-mesa-glx:i386.
So, I think another possibility is to add a dependency on libgl1-mesa-glx in primus-libs (and in the PPA too).
I inspected the dependencies of the virtualgl* packages in the PPA.
bumblebee recommends virtualgl | primus,
virtualgl recommends virtualgl-libs-ia32 (i386) (also depends on virtualgl-libs),
virtualgl-libs-ia32 (i386) depends on virtualgl-libs (i386 too, I think),
virtualgl-libs depends on libgl1-mesa-glx | libgl1,
virtualgl also depends on libgl1-mesa-glx | libgl1 and several other dependencies of virtualgl (redundant dependencies maybe?).
I suppose this is why installing bumblebee-nvidia from the PPA ends up also installing libgl1- mesa-glx: i386.
So, I think another possibility is to add a dependency on libgl1-mesa-glx in primus-libs (and in the PPA too).