The last relief valve is hard to get to, because it feels like you really have to force your way up to it. You do not have to force your way there, you have to relax and allow yourself to let the note come.
Developing vibrato is not something that can be written out in a few paragraphs. It's something that you can do by imitating a vocalist who has the type of vibrato that you want to emulate. The more you practice emulating vibrato, the more it starts to affect the way you sing, until it just starts happening by itself , but on command.