Airports and Connection Times

This is my take.
First of all, prices should be part of your decision too. Unless price is not a factor for you.
In any case, if price is not a factor, the best choice is always to minimize layovers and to fly directly from LAS to Europe. Since I think you mentioned that you can fly non stop (with Condor) from LAS to FRA, then FRA to FCO, with a return VCE-FRA-LAS. That is probably the best option for you.
Of all other choices, the better ones are probably the ones that minimize the number of layovers.
Another option you didn't mention is to fly from LAS to LAX then take the Alitalia non stop from LAX to FCO. Not sure if available when you travel since it's a seasonal flight.
Also British flies directly from LAS to LHR and LGW, so look into the flight option via London with British. British flies to both VCE and FCO.
Lastly, my suggestion is always to arrive to VCE first, and return from Rome. In other words, if you can, I would reverse the direction of your trip. This is because getting used to the jet lag in Venice is easier than in Rome. Also the VCE airport takes a long time to reach from the city, and if you have an early return flight, you need to get up in the middle of the night.

You didn't mention pricing. I just hope the airfares being quoted right now are favorable--vs. the airfares of Summer, 2015. We usually go the last couple of days of March, as Spring airfares usually go up on April 1st. If the airfares are not really good, you've got plenty of time to wait for the airfares to get cheap--or at least moderate. Remember, it all comes down to supply and demand--and the number of empty seats.

But there is also a quality factor involved here. I've never flown Condor, but they're part of Thomas Cook Travel. I would probably prefer to fly on one of the other scheduled airlines on such a long flight--preferably Lufthansa. While Delta's suffered some service problems in recent years, their JFK Terminal is a palace--and 1 and 2 hr. layovers are short enough.

It's nice that you have so many options on flights. Now, if they'll just get the prices down.

Thank you both, this is info I was looking for. Price could be lower, but the different options are pretty in line with each other so making it hard to make decision purely on price. I am hearing a direct flight from Las Vegas to Europe would be better and to look at options over Condor Airlines.

Any experiences you can share with going through Heathrow for passport control/customs, etc. that would rule out this connection, or conversely and other connecting city I didn't mention, like Paris?

You wouldn't go through immigrations or customs at Heathrow, only security.

Would only add that www.seatguru.com has cabin and seat maps for all of the airlines you're considering. I always check that site before finalizing our own plans...nice to know whether my quality of life on a long overnight flight might be improved if I book a specific flight (and airplane type) that offers a little more seat comfort (pitch, width, general cabin layout, etc.).

Both Virgin and BA flights direct to LHR are the best. They both have impeccable service. From LHR your options are numerous and it is only 2 - 3 hours to anywhere in Italy. We made the mistake of stopping on the east coast one time and it was awful. You just get settled in and then you have to deplane, sit in a terminal, and then get back on a plane for a long flight.