There is a useful article here about the care needed when interpreting raw numbers from VAERS: https://www.ibtimes.com/over-900-died-after-receiving-covid-19-vaccine-experts-say-data-misinterpreted-3153820
So the answer I think is no, they have not been investigated for possible causality, nor are there necessarily grounds for doing so. By the look of it, it may be the old post hoc ergo propter hoc trap.
It seems quite hard to find the data you are looking for viz. the risk of fatal side effects of the vaccine in fit and healthy under 30yr olds. I have been able to find the relative risk due to the virus of death, hospitalisation by age bands in the US, but this does not filter out the fit and healthy from the others: https://www.cdc.gov/coronavirus/2019-ncov/covid-data/investigations-discovery/hospitalization-death-by-age.html
I suspect that, for the cohort you are interested in, the main issues will be (i) the risk to them of Long Covid from the virus, and (ii) the increased spreading of the virus in the population by this group if they are not vaccinated - and hence the potential for further variants to arise.
I also suspect we simply do not have data on all this, given that the disease has only been with us for 18 months and given the huge number of variables, e.g. there are more than 5 vaccines in widespread use, there are half a dozen variants of the virus, all with their own characteristics, etc.
The only further thing I can contribute is that, in the UK, people under 30 are recommended not to have the Astra/Zeneca vaccine but one of the others, due to the low but finite risk of blood clots. I would think the same applies to the Johnson & Johnson one, as that, I gather, uses similar technology.
So Pfizer and Moderna seem to be fine for younger people. And by being vaccinated you do your bit for society by stopping the spread and reducing the chance of further lockdowns due to new variants. Plus you avoid the risk of long Covid, like my 25yr old nephew, and which I myself have had a brush with, as a fittish 65yr old.