VM core can be observed after reboot of a device, if the upgrade was not done using force-host on QFX5K platforms.

root@LAB> show system core-dumps no-forwarding

-rw------- 1 root wheel 103044227 Oct 19 16:12 /var/tmp/vmcore.0.gz

lrwxr-xr-x 1 root wheel     11 Oct 19 16:12 /var/tmp/vmcore.last.gz@ -> vmcore.0.gz

total files: 2

#cat vjunos-install.log 
========================================
/packages/db/pkginst.64226/install_script.sh /packages/db/pkginst.64226/jinstall-host-qfx-5e-x86-64-20.4R3-S3.4-secure.tgz
Downloading Junos image package...
Installing /packages/db/pkginst.64226/jinstall-host-qfx-5e-x86-64-20.4R3-S3.4-secure.tgz
jinstall_domestic_stripped flag is set to 0 for pkg:jinstall-host-qfx-5e-x86-64-20.4R3-S3.4-secure.tgz
RETAIN_PKG_IN_PREINSTALL_PKG_PATH flag is set to 0
Install media package found at /var/tmp/preinstall/install-media-qfx-5e-junos-20.4R3-S3.4-secure.tgz
Install completed
Pushing Junos image package to the host...
Installing /var/tmp/install-media-qfx-5e-junos-20.4R3-S3.4-secure.tgz
Extracting the package ...

============================================
Current Host kernel version : 3.14.52-rt50-WR7.0.0.9_ovp
Package Host kernel version : 3.14.52-rt50-WR7.0.0.9_ovp
Current Host version        : 3.1.0
Package Host version        : 3.1.0
Min host version required for applications: 3.0.7
Skipping Host OS upgrade!                  >>>>>>>>>> host upgrade issue
============================================

Upgrade the device with force-host option.