If I had the volatile ice relic (acid flask base damage converted to cold 100%) but then in the acid flask tree I selected explosive flask where its acid flasks base damage is converted to fire - which one would end up winning? I dont have a character to test with or I would have just done that but figured someone here will definitely know!
If the cold wins and i spec into the fire tree would I scale fire damage as it’s easier to convert to cold?
I was majorly confused because I didn’t remember the exact name of the item and assumed oyu did something from with the name. But what you are talking about is not a relic, it is a Off-hand Catalyst 
General rule of thumb overuling from left to right: Items > Passive > Skill Spec Tree.
I can’t confirm that this is always how its works, but that is how it works in the majority of cases.
Just to be sure I tested this on your specific example too make sure my above mentioend ruel of thumbs holds true to that and it does.
Haha omg sorry!! Offhand/relic get me all the time still!
So the item over rides the passive which over rides the skill tree - if I understood that right so if I used the “offhand” lol and it was all cold damage and I spec’d into the fire nodes would I then stack fire damage or cold? Fire seems easier to come by least for me so far lol
Because the Vial of Volatile Ice is an Item it overrules any other conversion, so it will be cold.
Thank you so so so so much <3
LE handles damage conversion differently from PoE. (If it was PoE, I supposed it would work just like you described).
But in LE, when you convert to said element, now you have to scale only the resulting element, completely ignoring the base element the skill had.