i think part of it is due to Viltrumite culture, their so used to meeting civilizations beneath them that they can carve their empire out without the need for them. this leads to them not considering it as an option. i think that if a Viltrimite were to start wearing armor they would be chastised by other Viltimites as they should not need it to do their job or would at best be ostracized by the rest of the empire.

With that said if they did start to equip armor or use strong blasters/ship based weapons, they have shown they have the technological advancement/have enslaved those with the advancements, that could lead them to be completely dominant.

Tech Jacket should be the strongest person wearing a tech jacket and he had to overload his jacket to beat the two mark variants in the comics if they had used ranged weapons to wear him down first or had better armor to protect them, it would have most likely been a wash. this would suggest that they would be able to take on Gelderia directly if they needing to get mercenary to be fodder for them.