It depends where you want to mount the tritium. I usually use an existing recess in order to protect the vial, and use Araldite Glass and Ceramic epoxy. It dries crystal clear and doesnt go yellow with age, which is what you want. I think most members in America use Devcon crystal clear, but i couldnt find that in my area. If you are mounting in a recess all you need to do is apply a TINY dot of glue where you want the vial, just enough to seat it in, and then sit the vial in it. I try not to get any on the visible side of the vial because it refracts the light and makes it look untidy, in my opinion, but it would make it stronger if you encase the whole vial in epoxy. Hope this helps.
Marc