I'm trying a Cowart muscadine in a 10-gallon fabric pot filled with the 5-1-1 mix. As you can see in the linked thread, fruitnut has had great success with 10 gallon pots, and that's about the limit of what I'm willing to do right now since I want to be able to easily move the pot. If you don't need to move the pots, though, I'm sure that your muscadines would appreciate a bit more leg room as time goes on.

As for your other question, I really don't see a compelling reason to chose the gritty mix over the 5-1-1 mix when you're using fabric pots. In fact, thanks to its enhanced breathability and wicking properties, I think you could probably get by with an even heavier mix.

Here is a link that might be useful: Growing Grape Vines in Containers

Thanks, shazaam. I'll give the 10 gallon pots a go.

As an aside, do you think the gravel show in the image below would work for the gritty mix?
{{gwi:35234}}

I would think so. Some of the pieces might be a bit large, but, overall, I think it would suffice. If you have a local Tractor Supply, their bags of granite chicken grit are ideal.