EDIT: Thanks for your help. I realise the original post was not very clear...
When using the system layout I have no problems using the default VS Code shortcuts. When I configure multiple non-system keyboard layouts (I am using the xfce4-keyboard-settings in XFCE) the defaults shortcuts no longer work.
This appears to be system-wide problem. When I create a dummy application shortcut using the "Application Shortcuts" tab in xfce4-keyboard-settings I can successfully create a shortcut using more than 3 (i.e. 4 or 5) keys if using the system layout. This is not possible when using non-system keyboard layouts: the maximum possible keys for a shortcut is 3. I have tried this on two laptops with different layouts, one using Xubuntu 18.04 and one using Linux Mint XFCE 19.2, and confirm this problem -- there is a limit on the number of key presses being registered using non-system keyboard layouts.
I have limited understanding of how keyboard layouts are handled by default in these two distros but it appears there may be a bug with how non-system layouts are handled. Any help would be greatly appreciated.