Yes it's very normal to wait several weeks after reference checks before a medical. Another reason for a delay may be that they have had trouble contacting one reference. Please be patient because
VB are fantastic about keeping you in the loop. As soon as they have news to tell you they will contact you. They move very fast once they have the go ahead.
There may simply be admin issues that they are resolving behind the scenes and I imagine it's not easy to co-ordinate the recruitment and training of several hundred crew.