Is vipassana outside a ten day retreat possible?
This is possible but anything less than 10 days for you 1st sitting may not give you visible results for an average novice. Also anything shorter may not have the critical push to give results. This is the reason the courses are organised in such away.

Also some continuity or length of practice takes you deeper and give better results. Just when you practice playing a game, more frequently you do it better you become.