Hello gussiejs.
I'm afraid that I don't recommend downdraft ventilation for gas cooktops,
especially for those as powerful as Viking, because downdrafts are not as effective as hoods. But there are many low-profile versions available that look wonderful, if you decide to reconsider this option.

First of all, when it comes to good kitchen ventilation, the hood's "capture area" should extend beyond the width of the cooktop by at least 12" (6" per side).
For example, a 30" gas cooktop should have a 36" wide vented hood.
This cannot be achieved with a downdraft.

Also, the rule of thumb is to provide adequate ventilation not only for cooking odors but for gas fumes as well.
To do so requires adequate CFM (Cubic Feet Per Minute) power of the vented hood, as it relates to the cooktop.
To arrive at the proper CFM for your cooktop, use this formula:
calculate the cooktop's BTU rating by adding the power of each burner together.
Then, divide the total by 100. This will give you the CFM requirements of your ventilation.

While I realize that this may not be the answer you were looking for, based upon your question, safety is paramount when it comes to responsible kitchen design - above & beyond aesthetics. I would be remiss if I did not give you my honest, professional opinion.

In closing, I wish you the very best of luck with this!
~Lynn

When I built my custom home I did a electric 6 burner cooktop with a pop up exhaust fan behind it. While I really like the fan dropping down into the counter.....It is not strong enough to pull steam, smoke or odors out. The power is almost equal
to a bathroom exhaust fan. Just make sure yours is industrial enough for a Viking stove. Otherwise love the clean drop down feature .

I agree with LM Designs. The only effective downdrafts I have ever seen is Jen-Aire. You must have a solution that is going to clear out the unburned gas and an overhead vent-a-hood is the most effective solution.