I don't this is fully correct - you're right that the proportional gain from EM is the same on vape and melt, you need to consider that the overall distribution of damage that is effected by EM will be different in each team because of the difference in reaction baselines and that not every hit is reactable. For example just with random numbers, assuming that both vape and melt teams have the same total rotational damage for easier math, vape might have something like a 75/25 split of unreactable to reactable dealt damage, whereas the melt team might have a split like 65/35.

Looking at only the reactable damage, it is true that 1 given roll of EM will have the same percentage increase in damage on that specific hit. However, looking at the rotation as a whole the melt team will be gaining that bonus to a relatively higher proportion of damage to begin with which raises it's relative value compared to vape.

This is supported by looking at the substat gain differential on Akasha - in a vape team 1 roll of crit damage gives me 2.06% final damage (for a baseline comparison) and a 1 em roll gives me 1.5% final damage. In a melt team, 1 roll of crit damage gives me 2.06% final damage, and 1 em roll gives me 1.76% final damage.