Though severe cases are rare after covid vaccination, now that there are more mild breakthrough cases due to the delta variant: is it less likely for vaxxed mild cases to become long haul?

Not sure I fully understand your question, but here goes: Breakthrough cases in fully vaccinated pts can occur, as success is 95%, not 100%, and some of these had only one JAB and did not follow through with number 2. The Delta has been blown out of proportion and is a risk in the unvaccinated mainly. COVID-19 Long appears to be a complication associated with a prior active infection.

The complications following COVID 19 infections which occur during the recovery period may be associated with any severity of the illness and present in a variety of ways. These include cardiac and neurologic conditions.