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.
Your motion isn't very consistent. After two or three oscillations it seems like your arm is having a spasm and ends in a very narrow shake. Both finger joints should be strong enough to not collapse, but flexible enough to bend when moving back and forth; while I cannot see the first joint, the second might be slightly rigid.
I suggest using a metronome. Set it at 30 or 40 bpm, do one full oscillation per beat, let it be wide, slow, and fluid, the idea here is get a slow continuous movement After feeling comfortable with the movement, do two oscillations per beat, the 4, etc. It's ok to do let the oscillations be more narrow when increase the oscillations per beat.
@Ferenc Simon Thanks for the feedback I will slow it down and try to nail the motion. The wrist vibrato was what I originally started with and it was difficult for me. I will give it a shot once I have more confidence.
@Gabriel great observations and I really appreciate the feedback. I will work on using the metronome.