try checking if you are running out of memory:
at the terminal:
this output will show you if you have any memeory avaiable (do this while openfire is running, try repeating it a few times to see fi memory is changing (balooning or something).
Also check to see if you have a swap file setup, as this will help prevent the kernel from forcfully killing your stuff when it’s starving for memory.
the same command of free -m will have a line at the bottom labeled “Swap:”, see if it has numbers in it or if it’s just zero (0). If it’s zero, you dont have swap setup.
Some VPS platforms do not allow swap at all, such as OpenVZ. To my knowledge, many others do such as Xen (XenServer), KVM, etc. However, the OpenVZ VPS’ tend to be the cheapest options (because they are poor in performance and have limitations such as no swap space).
To add swap, follow this guide: http://www.cyberciti.biz/faq/linux-add-a-swap-file-howto/
If your system has less than 2GB’s or memory, i’d say set at least 2GB’s of swap. there’s plenty of discussion on the web for proper swap space size, but start with 2GB’s and see what happens.