[Paul M]

From the FAQ (http://www.vbulletin.com/en/faq/)

What PHP versions does vBulletin support ?

Below are various versions of PHP and their supported vBulletin versions.

PHP 5.2 or older
PHP versions 5.2 or older have no supported vB versions.

PHP 5.3
vBulletin 3.8.7, 3.8.8, 3.8.9, 4.2.2 (No vB5 versions)

PHP 5.4
vBulletin 3.8.8, 3.8.9, 4.2.3 (No vB5 versions)

PHP 5.5
vBulletin 3.8.10, 4.2.4, 5.2.4, 5.2.5, 5.2.6

PHP 5.6
vBulletin 3.8.10, 3.8.11, 4.2.4, 4.2.5, 5.2.4, 5.2.5, 5.2.6

PHP 7.0
vBulletin 3.8.11, 4.2.5, 5.2.4, 5.2.5, 5.2.6

PHP 7.1
vBulletin 3.8.11, 4.2.5, 5.2.6

Note that PHP versions prior to php 5.6 are now E.O.L.
Any vBulletin versions not listed above are not supported, and their use should be avoided.