Hi Tim! We choose the Vanguard in preference over the Grandee for a couple reasons:
1) Space is a moderate issue - small back yard. The Grandee would fit but we did not want to go beyond the footprint that would fit our needs.
2) Primary use (we thought) would be two of us at a time. We have three kids but they range from 14 to 18. We figured on occasion we might all be in together but it would be rare. (Trust me, with teen schedules it is indeed rare.) Also, we are right on the verge of kids moving away for college so they'll be here less & less.
The Vanguard has no problem with all 5 of us in. Can you lounge, stretch out, etc? No, but it seems to work fine. We've all been in only 2 times in the six weeks we've had it. 80% of the time it's just 2 (my wife & I or one of the kids & I.) With 2 people the tub feels spacious. Same with 3 (that happens 1 - 2x per week.)
With the age of your kids, I'd guess you'll have a bigger crew a lot more often; and you have quite a few more years with everyone at home. You should set up a wet test with you HS dealer for both tubs and take the whole crew. Spend quite a bit of time in each tub and see if the VG feels too cramped. This is about the third thread I've seen the "why would anyone buy a VG when. . .) We absolutely love the Vanguard and it fits us perfectly. Don't let anyone else push your decision. Go try both and have fun. Hey, they are excellent tubs and you will be so pleased when it is right out your back door!