Our Forum is running on a high specification server but when we use mass mailing feature to email all 1000 members then the load goes above 15 - 20 for few seconds. If this can happen to a high specification server then your
VPS Hosting is expected to shutdown if you have more than 1000 members.
We prefer to send emails through our billing system as it sends 25 emails in one go and then waits for 2 seconds before processing batch of next 25 emails. It takes some time for the billing system to send emails this way but it manages to send all emails without any problem and other applications don't need to suffer due to emails going out from the billing system.
You should find some addon for your vBulletin forum to limit flow of outgoing emails. We have some customers who have considered
Dedicated Server simply due to these limitations of vBulletin forum software.
vBulletin overloads MySQL server and email server at same time as the mailing list is fetched from the database and then sent out using mail server. I'll communicate with our
VPS Hosting team to see if we can do something to get this problem sorted without disturbing your existing setup.