It is worth noting that the USD-priced version of VUSA, called VUSD, is already available on the T212 platform. I expect VUSA.DE might become available in future. Meanwhile, one could use VUSD.
Typical bid offer spreads can differ according to popularity. According to data on the Hargreaves Lansdown site, which is approximate only, VUSA.L spread is 0.03% whereas VUSD is 0.10%. Spread tends to be smaller where there is greater trading volume. According to Yahoo, yesterday’s volume for VUSA.L was 213k shares, and for VUSA.DE was 56k shares. Both dwarfed by VOO on the NYSE which was 1543k.
All versions of VUSA have base currency of USD, same ISIN: IE00B3XXRP09, are domiciled in Ireland and have 0.07% fee so are essentially identical except for currency units and popularity. This morning so far the exchanges are reporting that there have been about 100 trades of VUSA.L, 60 of VUSD on LSE and 20 of VUSA.DE on XETRA.