If staying in Fort Vieux is this an are prone to a lot of mosquitos. are there cetain months when the mosquitos are most active and when are they more passive.

Yes, lots of mosquitos, especially around Coconut Bay as they have extensive gardens. Mosquitos are more prevelant after decent rain and around heavy vegetation and in the morning & evening hours (5-7am & 5-7pm) Off! works great and is available on island - about $10US inthe drugstores in VF.
Enjoy.
We went last feb and had no problem with bugs they do spray every morning and at dusk u will see the fogger tractor.
Can I suggest a trip to the pitons for snorkeling even though coconut doesn't offer any more just talk to the guy at ft desk.
U may need to walk down a very steep drive way but well worth the effort. Enjoy ..