This is possible to do on your own by creating a custom BB-Code in PHP and using a wrapper to VaultWiki's headline handler function.
Also, NuWiki had separate tags for H1, H2, H3, etc, where the code was more like:
Code:
[h2]{param}[/h2]
[h3]{param}[/h3]
If this is what you're looking for, you can enable NuWiki-style BB-Codes via your AdminCP in Settings > Options > VaultWiki: Legacy Options > Enable support for Legacy Wiki Markup. This will enable a number of deprecated BB-Codes (only in wiki pages) and will slow down page generation time.