Lisa,
Thanks so much for your question!
As it happens yes, you are likely to be able to use your Disney Dining Plan at the Epcot International Food and Wine Festival. While it has not been officially announced, in recent years the booths have had a few different items each that you could get by using your snack credits.
For example last year my favorite booth, Belgium, had both the mussels and the waffle available in exchange for a snack credit. Plus these were their two marquee food items. In fact I think that one of the best ways to use the Dining Plan snack credits is to have a dinner made up of little snack credit redemptions at the festival.
As for the stands staying open through the evening Extra Magic Hours, that is a bit iffy. Technically I think that they do, though I find that many will close prior to the final hour or two. I would not wait until the last hour if you have something you would want to try.
Send me more question! I love chatting about food!
Have a magical (and delicious) vacation!