I know there are some standards that are CISCO proprietary but have become open standards. CISCO has also updated the RFC to reflect this. So, is this also true for PVST?
Does Juniper or other vendors support PVST or not?
(R)PVST(+) is a proprietary standard defined by Cisco (or a set thereof). However, there are many other vendors and devices supporting it - you may need to check the specifications of your devices.
The IEEE standard alternative is MSTP which enables separate spanning trees for different MSTP instances. Additionally, MSTP also supports splitting a network into separate regions or domains. MSTP is more complex but also more powerful than PVST.