I kept thinking about your post this morning while out on the trails with Gracie (5 1/2 months old).
The line that struck me most was "as well as play for hours with our dog-loving children".
One of the most important things for you to understand going into this is that your girls will not be exercising the dog in any meaningful way. Unless your girls are accustomed to spending over an hour EVERY day (rain, snow, shine) hiking in the woods, that baseline need will need to be met by you or your husband.
My own kids are admittedly younger than yours -- they are 8, 6, and 4 -- but I think our experience will still be relevant to you. Pre-Vizsla, I thought of us as an active family that loved to spend time in the great outdoors (and by any reasonable measure we are). Still, I have found myself needing to be in the woods ALONE with Gracie every day while they are at school, because I cannot drag the kids out for an hour walk in all weather every day, especially as it gets colder here in MA. They do not have the stamina or the inclination.
We DO get outside all together with Gracie, and these outings have formed some of our best memories of our first fall with her...but the kids max out at one or two big walks per week. I don't push it because I want them to love the woods, not resent it because they were dragged kicking and screaming for the good of the dog.
A Vizsla NEEDS, NEEDS, NEEDS to spend off-leash in nature. Gracie is curled up on the sofa next to me as I type this message. She is a happy girl today, because she spent her morning racing down the trail, bounding across boulders, flying over the trunks of fallen trees...shooting through the underbrush, down to the riverbank, up over the crest of the hill...zooming out ahead of me and then zooming back to check in before zooming away again. She lives for this kind of activity.
For the past two weeks, we got to see what Gracie is like without spending adequate time in the woods. She developed an abscess under her jaw that required surgery, and our vet told us to keep her out of the woods while her stitches healed. We still played with her in the yard. We took her to an enclosed baseball field and threw a ball around every day. She. Was. Climbing. The. Walls. She. Was. Eating. Everything. In. Sight. Digging. Barking. Jumping. Aaaagh!
Playing in the park was absolutely NOT a substitute for doing what she loves.
Now...if Gracie does get her needed time off-leash in nature, then the time she spends with the kids is absolutely golden. They goof around in the yard, play with her toys, etc...but most wonderfully of all, they snuggle with her! And she is one happy Vizsla -- curled up on the sofa under a blanket on my oldest son's lap while he is reading, or sitting with my middle boy in front of the heat vent while he fiddles with his LEGOs, or sprawled out on a bed with my daughter rubbing her belly. It doesn't get any better, for Gracie, for my kids, or for me.
But plan on that nature time. Every day. Your V won't be happy without it (and then neither will you!).
I've never owned a lab, but my hunch from watching others is that
lab + ball + someone to throw it = happy dog
lab + kids to run, chase, and play with = happy dog
I don't think the same equations hold true for Vizslas. They need more. Giving it to them is good for your body, mind, and soul...once you figure out how to fit it into your day.
Hope that this message is helpful to you! Good luck!