The wiki stats this for Heavy Caliber:
"Instead of reducing the accuracy by 5% per rank, each rank increases spread by a specific percent per rank depending on the weapon. This effectively reduces accuracy but may reduce it by greater than 5% per rank (according to the Arsenal UI)."
and this for the Vectis:
"Pin-point accuracy, shots will hit exactly on the reticule dot, perfect for headshots."
Which made me wonder if there is any negative influence on the vectis at all, as it apparently has no spread. Need to level up Heavy Caliber to try it out myself.
I just tested it and using heavy caliber does deviate your shots. It seems a bit random as to how much, sometimes they hit dead, there was minor deviation on some of a fraction of a degree. That might not seem like a big deal but at long range it is magnified. With only one shot I'd say Heavy Caliber might not be not the best choice.
It all really depends what range your at mid-range its defenitly fine, but missing shots pisses me off which is why I have my vectis modded to have 2 capacity clip. admittley it's really pricey for mod points but I like the added flexibility
Nekroing a bit here, but doing recent testing for Heavy Caliber and it's definitely a bad mod for vectis.
HC and Serration stack additively meaning that the damage increase from having Serration alone to HC+Serration is around 60%(with maxed HC it's 430% / 265% ~ 162% ) while suffering heavy accuracy penalty. For the exactly same reason putting HC below rank 7 would mean that you are actually hurting your weapon instead of improving it(gives less than 40% total damage increase). It's better to put charged chamber and go for headshots - you get only 20% less damage but for almost half the power cost and ablity to actually shoot the head off.
Heavy caliber is a BAD MOD for this weapon. This weapon is a sniper and is very role specific. Instead of taking quick shots where ever you can land, you attempt to amplify the massive sniper-based damage by taking a little more time to land a hit on the enemies weak spot (head shot for example). Heavy caliber's assassination to the accuracy of the weapon conflicts with this playstyle and will severly damage playstyles of good snipers.
Simply, this sniper is almost always a one hit kill. This is why having high crit chance is great and having low status chance doesn't mean anything. They SHOULDN'T live longer enough for your toxin or fire or slow to take proper effect, that's the goal of having a weapon like this. Prime your Vectis up by adding extra elements to free spots and over your Heavy Caliber mod because the gun already has excellent all round damage but you can boost its weaknesses or strengths a little more with appropriate elements. You don't want the elements for the status effect but instead the damage to specific units (example; viral versus Grineer).
Basically: No heavy caliber. Use other mods that help the UTILITY of the weapon or amplify it's specific playstyle; the ability to pop off heads ACCURATELY. If you're dying to use heavy caliber, don't play a role as a sniper because your damage will suffer drastically.