I found an interesting starting seed for vaulters (5 hot planets), but... the (random) galaxy seed number is negative.

There's no way that I know of to actually enter a negative seed number when starting a new game.

This bug has existed for at least 3 years.

Why not just finally allow the negative character (hyphen) to be entered into the seed field dialogue box for user input?  It's such an easy fix.

You could try editing the seed field with cheat engine or something, also, in the xml I believe you can change the min/max values of the number entry.