Users are not receiving reset password emails

[snakes1100]

Unfortunately to truly trouble shoot this issue, you need to tail your maillog.

ie... tail -f /var/log/maillog
cpanel - tail -f /var/log/exim_mainlog
plesk - uses qmail and depending on what plesk you have on the server, its log file could be in one location or another.

You could also try enabling the smtp option in the admincp --> vbulletin options and try that route.