This is exactly what we did however we did the following:
1) Put Riu on a bigger server
2) Upgaded the database
3) Upgraded php-fpm
4) Installed litespeed (apache replacement)
5) Installed a master and slave db
6) Installed a load balancer
7) Installed another web server to handle down-times in case we need to take one server down.
Got rid of Vbulletin Search Completly and integrated Sphinx Search into our system.
9) Still dealt with all you people whining
10) upgraded vbulletin and had to fix styling issues because they changed their html generation code.
11) Spent more time dealing with people whining.
So all in all it went pretty smooth for the upgrade that we completed.