This article clarifies whether VSTP frames are tagged or untagged traffic.

Customer needs to be certain about whether VSTP frames are tagged or untagged traffic.

According to the documentation below related to single-tagged access ports and double-tagged trunk ports, VSTP frames are tagged while RSTP frames are untagged. Although the topic is out of the scope of this query, it answers the question successfully.

Configuring VSTP on a Trunk Port with Tagged Traffic

In 802.1ad provider bridge networks (stacked VLANs), single-tagged access ports and double-tagged trunk ports can co-exist in a single spanning tree context. In this mode, the VLAN Spanning Tree Protocol (VSTP) can send and receive untagged Rapid Spanning Tree Protocol (RSTP) bridge protocol data units (BPDUs) on Gigabit Ethernet (ge), 10 -Gigabit Ethernet (xe), and aggregated Ethernet (ae) interfaces. The untagged RSTP BPDUs interoperate with tagged VSTP BPDUs sent over the double-tagged trunk ports.

https://www.juniper.net/documentation/us/en/software/junos/stp-l2/topics/topic-map/spanning-tree-configuring-vstp.html#id-example-configuring-vstp-on-a-trunk-port-with-tagged-traffic