Why is there a difference in dividend yield for VUSA and VOO?

Hello! I'm not 100% sure about my answer. But a possible reason is that VUSA is a fund domiciled in Ireland. For non-US investors (be it an individual or fund), dividends from US-listed stocks are subject to a 30% withholding tax. So in the case of VUSA, the dividends it receives from US-listed stocks may already be lowered by 30% of the US withholding tax - these dividends are then distributed from VUSA itself, resulting in lower dividend yield.

It's worth noting that the dividend yields you stated appear to gross yields, meaning that, what you eventually get from VUSA _still _has to be lowered by 15% to account for the 15% withholding tax for Ireland-domiciled funds. The 1.98% for VOO that you get will have to be lowered by 30% since it is listed in the US, and hence there is the US withholding tax to consider.