I keep getting sneak attack kills and yet the bodies arent getting disintegrated/goo-ified

has the mechanic changed?

I think so, i rarely see and sort of gooification anymore sadly :[

Disintegrations and critical hits are different things.

in the last game every enemy that you killed with a crit from an energy weapon would disintegrate/gooify

I think sneak attacks in FO4 are damage multipliers now. "Sneak attack for 2.5x damage!" Also consider you can only critical hit in VATS now. (I wonder if this would let them stack, and you could destroy an enemy with a sneak-crit. Disintegration/goo still always happens on critical deaths, but they now seem to have a chance on normal deaths too.

"sneak crit" is no longer a crit for the purposes of weapon mechanics, but you can trigger both simultaneously.

sneak attacks only do double damage or more up to 3.5 or more multiplier with the right perks correct me if i am wrong so with a fully modded sniper rifle and all the perks you can oneshot just about everything when it is a sneak attack

if you want criticals you need to go into vats and activate one if your critbar is full

Don't use energy weapon much but pretty sure I have made goo without VAT.

Know that npc and even turrets can make goo.

I rarely use VATS or plasma weapons, but I burn things to ashes almost every other kill with regular laser fire. And like it's been said above, crits and sneak attack multiplier are separate things, you have to manually activate a crit from VATS now, and you can only do it if your crit bar is full. There's also a perk that lets you 'save' a number of crits for use later. I'm pretty sure you can stack a crit on top of your sneak multiplier if you have one available as well.