Hi Lindalk, welcome to the board
Deck 11 is a good choice as it has cabins directly above and below - hence no noise of buffet chairs/sunbeds being moved etc.
I would like to add about booking through a travel agent. If you choose one one of the "bigger" ones, ie one you might see on tv - always bear in mind the man doing the talking knows very little about the ships, and the people on the phones know even less! From my experience (and from what I have read here) they will tell you pretty much anything to get you to book. They are okay though if you know exactly what you want.
I have been through Bay of Biscay (generally day 2 of the cruise if leaving from Southampton) on many occasions, and have only really felt noticeable movement once, but like an earlier person said its like how long is a piece of string. Get your seasick meds just in case!
An important tip for any cruise on any ship is keep your important stuff (paperwork/passport, jewellery, meds) with you in a holdall. Do not put this stuff in your case because you may not see this again for several hours. You may want a change of clothes, a bit of make up and your swimming costume in this bag too so you can hit the pool straight away.
There is tons of information here about Virtuosa. She is a beautiful ship. I am sure you will have a lovely time.