I have a Viking 630 (old manual) with a vertical bobbin, and it does wonderful FMQ. I bought the fmq foot for this machine, and after a couple of months it broke. Not wanting to wait, I tried fmq with an open toe foot - it still did wonderful fmq, and has been ever since.
I discovered on another site, a bunch of people that had the same thing happen with the same foot (it can be used with other machines also) So that settled it, I'll do without. I think it was $40 or so, with shipping