I think they just look at what grades the kid has completed (whether homeschooled, private schooled, or public schooled in another district) and put them in the corresponding next grade up. They're not going to test for placement purposes for the most part. That said, in many states homeschooled kids are tested yearly. I wouldn't be concerned about how well these kids will do with a standardized test, though.
I've heard of parents having trouble getting appropriate work for their kids even in VPS. If you do go that route, I'd make sure you work out in advance exactly how you're going to get appropriately challenging work for a gifted kid who may work through the material more quickly than a ND kid.
