In my experience it’s always installed as you say, to “C:\Program Files\Steinberg\VSTPlugins”, while the VST3 version went to “C:\Program Files\Common Files\VST3”.
Just some more ‘ideas’…
Have a look for other possible/common locations for VST2 plugins like:
C:\VSTPlugins
C:\Program Files\VSTPlugins
C:\Common Files\VST2
Likewise for 32bit versions in C:\Program Files (x68).…
You could also try a drive search with Windows Explorer to scan your whole system for
Halion 5.dll
If you don’t find it anywhere, make sure you have the latest version of the installer and try again from that.
Downloads for installer packages and updates:
It might be possible that some installers might only give you the VST3 version unless you take extra steps to get the VST2 version as well. If you don’t see anything special about this in your installer…and you find that you do get a VST3 version installed but no VST2…then check the ‘Extras Folder’ of your installer package.