VSTP was delveloped by Juniper to provide inter-operability with Cisco PVST+. For R-PVST+ interoperability, enable both VSTP and RSTP on the interface - yes this combo is allowed.
If plan is to interoperate STP between Cisco and Juniper, you MUST know what version of STP is running on any Cisco interface that connects to Juniper.
In general, there are many better options than to run STP in today's networking.