yearn-exporter VS defi-score

Thank you kindly! That's a great resource (yearn.vision) - appreicate it!

You can manually calculate it by looking at the change in share price on yearn.vision between the date you deposit it

You can find some here, if you're willing to do some digging: https://yearn.vision/

To see what is the "real" current APY (and not the monthly-averaged one, which makes no sense) i recommend to look at yearn.vision dashboard, and more specifically the one about APYs : https://yearn.vision/d/CRDOk-UGkkkd/apy-technical?orgId=1 just find your vault's name and you can set the calculation based on the last few hours or days at the top of the page

But given the data available from yearn.vision is it possible to deduce/calculate the performance / earnings over time (as shown on yearn.finance) of a given yVault?

Differences can be due to several factors. I am assuming Yearn.vision relies on historical data and uses no extrapolation/predictive analytics or other statistical analysis on the value timeseries. It is unknown to me what the oracle/data source of yearn.vision is though, but a look at https://github.com/yearn/yearn-exporter should answer that.

You can't account for the unknown unknowns, but for the rest you can begin here: https://github.com/consensys/defi-score