VST 2.9 is compatible with CompCert 8.13, 8.14, or 8.15; and with CompCert 3.10.
The simplest way to install VST is to install the Coq Platform.
The Coq Platform 2022.01 already contains VST 2.9 in the 8.15~beta1 package pick.
If you install Coq Platform via the "coq_platform_make.sh" script, it will ask you which package pick to install.
By mid-2022 you can expect that a non-beta 2022 Coq Platform will contain VST 2.9 (for Coq 8.15 and CompCert 3.10).
If you do not want to install a beta version of Coq Platform, you can also install the 8.14~2022.01 package pick via "coq_platform_make.sh". This package pick contains VST 2.8, but you can change this by first installing Coq Platform without CompCert and VST (it will ask) and then running "opam install coq-vst.2.9".