You guys already caught most of this stuff. Avoid Ryanair for all the reasons above plus the fact that if you have luggage you will probably end up paying extra for that too (their weight restriction is miniscule)--and it's a rather tight and uncomfortable ride from what I know.

It's unclear to me whether PAR is a stopover or not, and whether you have any flexibility on how you're going to do the trip.

Also, try to shy away from Alitalia (unless it's a codeshare of a flight operated by someone else): they're bleeding severely and their unions are doing everything to bankrupt them, including wildcat strikes.

On the way to/from FCO, unless you're stopping over at the intermediate city, definitely route yourself through LHR with a BA FCO-LHR flight, as you will get the connecting flight for "free" within your 40,000 miles award, which becomes an "open jaw" award--something that is permissible.

On the way to/from VCE, unless you're stopping over at the intermediate city, definitely route yourself through BRU with an SN FCO-LHR flight, as you will get the connecting for "free" within your 40,000 miles award, which becomes an "open jaw" award--something that is permissible. Note that flying through LON to/from VCE is not a good option, as you'd have to switch airports (a royal pain) and connections are usually not very good (overnight required).

If stopping over in PAR is a requirement, think of buying a roundtrip and throwing away the return: CDG-VCE is ~$222, while the cheapest one-way (although refundable and that automatically gets you in business class with hot food service and gets you into the lounge) is ~$845.