I got really excited when I first saw the model for these robots - they are a direct back-link and throw-back to the old Rogue Trader era robots!
In simple terms, these robots are the hunt-and-kill variety. Sleepless, tireless, but more insectoid in appearance, rather than humanoid. Cull human populations in times of famine, seek out degenerate machines on Forge Worlds, or put to war against xenos. They're equally effective.
The Vorax are both lighter and faster than their cousins. This is reflected in the fact they have scout and fleet. Set up in the right manner, this combination can lead to a deadly second turn assault.
More than this, their high toughness (T=6) coupled with 3 wounds each means that they have the staying power to overcome any shortfalls in their 4+ armour save. But that shortfall is a serious one.
As mentioned, the 4+ armour save is a real drawback here. But with careful positioning and a game plan, this can be mitigated. Their relatively low profile on the battlefield means that they should also be able to find cover regularly.
So what to do? My opinion is that they are just about viable, but I think one needs to examine their role carefully. I would be tempted to outflank a unit - either as a distraction or to take care of a back-line devastator squad. Rite of Celerity will also help with the I=4 issue as well. It just depends on how the rest of the army is set up.
All that said, I think a single model in one maniple is a good way to go here.
1 Vorax, frag grenades, irad-cleanser, paragon of metal (120 points)
Click and point, Sit back and enjoy the (programmed) result.
1 Vorax, frag grenades, enhanced targeting array (95 points)
Cheap, relatively speaking, and a bit of a distraction to enemy units.
5 Vorax, frag grenades (400 points)
Probably not worth while in terms of points expenditure. I include it here only as an illustration of what a large squad of these might look like. Don't play it. Use a single one instead (in my opinion).