1. Yes you can directly migrate onto your vDS. If you to it "right" you have a already existing Portgroup from type "ephemeral" because you cant convert a exising non ephemeral to a ephemeral later.
2. We use it permanently and this is how i implement it on all my customers since years. We start with vDS and vSphere 4 somewhere in 2010 or so.
Regards,
Joerg