1. VM data will not be lost in brownfield installation. Backups are recommended as you are already doing.
2. Role of cluster service is to ensure that VSA cluster stays up if there is one host failure. So with two hosts and one vC running cluster service your VSA storage cluster would still be functional with one host down as long as other host and cluster service is running.
To your point if cluster service goes down VSA should still be functional as there are two nodes in the cluster up (the third vote is from cluster service) however if you have had a host failure during the same time as cluster service was down then you would have VSA go down.
Hope this helps.