If they had meant for Stranglevine to only gain 2 life every Upkeep you pay for it, they could have just said "gains Life +2" instead of "gains Life +2X". Clearly the number of Crush tokens is supposed to determine how much life you gain. And since X changes every upkeep, it makes sense that you would have to resolve that effect every upkeep too.
Alternatively they could have said "Stranglevine's life becomes 6 + 2X". But again, they didn't, they said "gains Life +2X". Therefore the card gains 2 life, then 4 life, then 6 life, etc. as written. It is exponential as written because the Life +X trait has a + sign, meaning it stacks, just like the Melee +X trait and unlike the Regenerate X trait.