My approach has always been not to ask for permission, but be prepared to beg for forgiveness if you board takes off. If UBB.threads is causing a resource issue on your server, then you have a "High Class" problem since you will require several thousands of users before it will attract the attention of the host.
For the mail issues, especially delivery problems, I use Google Apps and gmail's SMTP servers. Each Google Apps account has a limit of 500 outbound emails per day, but I use a cron script to rotate accounts every 10 minutes. You can have up to 50 accounts on their free plan, meaning 25k outbound messages a day. If you set your SPF correctly to include the google servers and your host server then you will rarely have a problem with delivery.
One of the problems with VPS is security and the admin overhead/learning curve. I have used VPSLink for many projects, but their IPs are constantly targeted as the hackers know that these virtual boxes are mainly managed by inexperienced newbies. If you do go down the VPS path then always check the IP addresses you are allocated against the blacklists. If yours shows up then request another or you will definitely experience problems getting your mail delivered.