I don’t think this was a vbulletin upgrade. It looks like it’s just a new pre-packaged theme purchased from here. https://www.sultantheme.com/product/st-vb3-responsive/
For those not using phones if you’re annoyed by the giant font you can always set your browser zoom level to something more readable (control and + or - keys to zoom up and down) I find a setting of about 70% brings it down to a reasonable size. Unfortunately there isn’t a button to correct the overall ugliness of this theme. I hope this is just an accidental leak of the beta version or something and there will be many changes still to come.
Some of our userscript authors may be able to write a script to use the old theme instead if it’s still on the server. The current theme is called ‘Sultantheme’s Responsive vB3-blue’ and the old one was called ‘Straight Dope v3.7.3’ I’m not sure a simple replacement of the theme name will work. But if you’re a userscript guru looking to improve the world, you know what to do.