That's pretty much it You got arm vibrato down, now all it takes is doing this as much as you can without pain to train your muscles for endurance. Don't try to rush it though, speed up to where you mess up the timing, then dial it back just enough so you can do it evenly and keep it there, don't try to go faster.. If you do this every day you'll notice that this speed at which you can still keep it evenly will increase on its own day by day as your endurance builds.

What you could try in the meantime is to add a bit of wrist to it now and then (aside from your regular vibrato practice), just to start getting a feel for that as well. Using the wrist will allow you to do vibrato with much less effort at higher speeds later on.