One workaround, on Mac at least, is to use AppleScript applets to a) launch Obsidian and then b) invoke any obsidian:// URL command (open Vault, etc…)
(This will open whatever the last open Vault was first. No way around that I suppose.)
AppleScript code example:
tell application "Obsidian" to activate
tell application "Obsidian" to open location "obsidian://vault/Vault"
AppleScript can be invoked as little applets (use the “Script Editor” to make these), or as scripts via the Terminal (osascript /path/to/script). Raw AppleScript code can also be executed this way via Terminal, and any scripting language that supports calling osascript (shell, Python)
This is good enough for me, and opens other doors for exploration.
Cheers.