[Mark.B]

Originally Posted by kojay

I've dug deeper and can tell you where to look for the problem. This is related to the navbar. I would like to keep the standard style.

As above, the vBulletin 3 style is not a responsive style. It was written some years before viewing websites on mobile devices was even a 'thing'.

You will need to rewrite the style or at least substaintial parts of it parts of it using Responsive Media Queries

You also need to be, for the most part, removing the tables that make up much of the vB3 design and replace them with divs, as tables don't generally work well in responsive designs.

None of this is a trivial task and requires knowledge of CSS, html and vBulletin code.

The alternative is to obtain a third party responsive style and customise that to match the look and feel of your existing style, which would be the easier option in my view. The essence of many styles is the colours and fonts and those are quite easy to change.