Seems to me that whenever SeaMonkey is installed in VL, this issue ought to be dealt with automatically.
Can't argue with that, but the solution is easy. Just right click and choose 'open with' then 'open with other application'
Then click on the 'use custom command'...in the box type seamonkey.
And tick the 'use as default' box. I suspect the problem is due to the shared-mime-info package, which is a Slackware package, not a Vectorlinux package.
And while we're on the subject, everyone should be using Seamonkey 1.1.16, not the package in the title. There were security issues fixed with the 1.1.16 version. In fact, it looks as though 1.1.17 has just entered testing (not been released yet, so there isn't a Vector package yet).