Your ventilation needs will already be quite aggressive and expensive. With a grill, they become even more critical to get right---and even more expensive. If you live in a location where you can grill outdoors year round, it's a no brainer. Skip the grill. If you live in a cold climate, you need to decide if you want to spent more on your ventilation than you do on your range. Because by the time you add in the needed heated makeup air to kee from freezing your home to death or killing your family when you're running the vent, you've just spent closer to 12K on the vent than you have the range.

I am getting a 42" VAH, which most people on this board seem to recommend. Is that not enough?

I think the best option no matter what stove it is is always to get a 14x23" steel griddle instead of either a griddle or grill that is part of the stove.
That way you have a much wider option in cooking surface.

The Chef King griddles are the best by far.

They also have Grills, they call them Griddle/Broilers.

http://www.dvorsons.com/AddABroiler/AddABroiler.html

Here is a link that might be useful: Griddles

How many CFMs does your 42" VAH draw? I have a 36" 6 burner gas range, with a 600CFM VAH over it. The same range with a grill requires 1200CFM. My 600CFMs requires heated make-up air, and the entire vent installation cost me about $9500. My HVAC contractor said to NOT EVEN THINK about 1200CFMs

"I am getting a 42" VAH, which most people on this board seem to recommend. Is that not enough?"

We have had a 42"X27" 600 CFM VH over 36" AG range with grill into its 8th year. Overall has worked fine and rarely lets out a little smoke from grilling. Much of how it will actually work is beyond blanket CFM ratings/recommendations and dependent upon ducting, how the hood is surrounded and its capture area and design. Our kitchen area is relatively small and if the hood is on high the hood probably exhausts the entire kitchen space in minutes. We can grill outside most of the time but for inclement weather and just wanting to chat while cooking the grill is great. Given different cooking styles not surprising there are different preferences.

I would presonally go for the grill or griddle over 6 burners. I have used indoor grills for years and I find them very useful. Many love their griddles and I look forward to using my new one (I just installed a 48" CC with grill and griddle).

Even though I know I will be smacked down hard by some I will say I use a 1200 cfm insert and open the window next to the unit for make up air. I have only once had to turn the fan on high, most of the time it stays on the lowest setting. With the extra heat put out from cooking I actually like having fresh air come in from the window. For some this will be totally unacceptable, but it works for us and I have lived with 12-1400cfm vents for years and have never had a drafting issue with the fireplace going.

"I use a 1200 cfm insert and open the window next to the unit for make up air."

Same here. Shhh, don't tell anyone. Like you said, the 1200cfm draws plenty of air at the lowest speed, very rarely have to go higher.

-Stooxie

Where can I get a 1200 cfm insert?