Sorry wall of text:
Tldr; it doesn’t reduce dmg/healing to generate the vitality just generated based on the effective healing dealt. It’s a buggy spec. The spreading part of coalescence is a lie. The intensify part is a chance on direct hit to DOUBLE the existing hot/dot and eats your vitality. It’s not free, just helps you dump it faster.
Ok, so I’ve spent a ridiculous amount of time on harmony for both brew and mw to solve it like the frustrating puzzle that it is.
A lot of the wording is confusing. It’s also riddled with bugs and has poor vitality tuning for both. So…
First thing to get out of the way is that the target dummies are weird and seem to take reduced dmg and healing from you so take dummies with a grain of salt.
For MW it’s 30% of your effective healing generates vitality (won’t take from it), and half that for overhealing. Then 10% of your effective damage. Last time I tested it I remember it being like 40% not 30% and 20% for overhealing and I think damage was also higher.
Spending vitality is where it gets weird. When it starts, you won’t generate vitality and it won’t end until 10 seconds pass or you spent all vitality.
The dot you apply is based on 40% of the initial dmg or healing you did and consumes that much vitality and stacks in intensity and refreshes the duration. You do 20% more DMG or healing to someone with the dot/hot. BUT, there are caveats.
It works on a whitelisted spell system, so only specific spells will be do 20% more against the dot/hot. Same for which spells can spend vitality or generate it. So for mistweaver (with dmg spending talented) this is what I know of:
Procs, trinkets, non monk spells: don’t generate, don’t spend, aren’t boosted.
Majority of intentional healing will spend vitality, such as Gust healing, renewing mist, env mist, vivify.
Jadefire Teachings fistweave healing does not interact with it last time I tested. It won’t spend vitality or generate it or get boosted by the dot OR hot.
Rushing Wind Kick is also a new spell they forgot to add to some whitelist apparently, be cause despite replacing and inheriting rising sun kick, it doesn’t spend vitality. But it does generate it and gets dmg boosted.
Now for coalescence, the spread and intensify parts are very unclear. The spreading part never happens on MW, I’ve tested everything to try to get it to. The dot for brew/mw NEVER spreads to nearby enemies/allies. Only brew can have the hot “spread”, and only when it’s on them. Then when it “spreads” it’s just spending your vitality to apply another hot to allies equal to the current hot on you. It’s not vitality free.
Same goes for “intensify”. It’s never free, and is only a chance to happen on direct dmg hits. I want to say roughly 25% chance? And what it does is, when you hit an enemy that has the dot already and it procs, it will DOUBLE the existing dot on that target and consume vitality from you equal to that amount.
This spread/intensify thing only happens during the spending window too. The intensify part is important to know because it will determine if you completely waste vitality by not spending it fast enough or spend too much on the wrong target.
So for example as mistweaver, trying to use vitality primarily for dmg. When you pop tea with full vitality. Your existing hots will consume a tiny amount to apply the harmony hot on people and give you that 20% bonus healing. Then you start TP/BK/RSK the targets you want to dump vitality dmg into and keep dpsing until it’s all spent.
You could tiger palm one target and dot them and nothing else, but fail to spend all your vitality in time because you’re not hitting fast enough or are unlucky. What you can do is tag the targets you want to prio dot dmg and just spinning crane kick until vitality is depleted because it hits fast. You’ll notice vitality dump exponentially and instantly sometimes because the intensify procs are DOUBLING the dot each proc and eating that much vitality. (This was the one shot bug in pvp at launch, it didn’t spend vitality to double lol).
Conversely you may want to get the dot on as many targets as you can for the dmg amp for crackling dmg or sck dmg etc.
Sometimes your intensify procs could eat all your vitality if you gust heal or vivify allies and may not be what you want, but just so you understand what it’s doing.
For brew it’s a lot buggier and weaker cause they generate so little while mw generates too much lol.
One last thing, the harmony dot/hot itself won’t generate vitality when out of the spending window, and the bonus dot/hot talent where it dumps the remaining ALSO won’t generate vitality nor get boosted by coalescence. So double casting tea or celestial Brew to dump it all won’t so anything extra for you.