Bit of a tricky question, the way it's supposed to work is the default application is the SolidWorks Shell Launcher that scans the file for file version and if it's a 2018 file opens it in 2018, but if it's a 2017 file and you have 2017 installed it will ask you if you want to open in 2017 or 2018. That very rarely works for anyone though haha, it's pretty easy for that to get reset. I think you can restore that setting with a bit of brute force though by right clicking SolidWorks 2018 and running as an administrator, then going to options, find the setting under File Explorer to "Restore File Associations" but don't click yet, bring up task manager (ctrl+shift+esc), find the explorer.exe process and kill it (not restart, needs to be dead), back in options hit the "Restore File Associations" button, in task manager go to file -> new task and launch explorer.exe again to bring your desktop back.
Just tested and here is what I found
If I have a SW app running, file from VaultView is open to the running version
Without any SW app running, a dialog box pop-us asking to select SW version