Not sure which product you are talking about, but yes MC-LAG configuration can be used with type virtual-router or virtual-switch. Without doing something special, generally route leaking, but default VLANs in the different Virtual routing/switching instances can not communicate. Generally this is accomplished via some extenal FW/SRX. In this way VLANs within an instances can communicate directly (route) but between instances need some policy (FW rule) to communicate. This is used very often and is quite common.
With a VR/VLAN if you want MC-LAG A/A with local routing on each switch regardless of which link the external (LAG'd) device takes, then yes VRRP is required. This is not the case with VC. The RVI would span the whole VC depending on which interfaces are members.
Hope this helps.