[objclxt]

> (Germany or UK in my personal case) [...] not sure if Vanguard is accessible to non-US citizens, and there might be tax implications in different countries

Vanguard operate mutual funds in the UK priced and traded in GBP, through their UK subsidiary. They're available through most major trading platforms. The one thing to note about Vanguard's UK funds is that the management fees are higher than their US equivalents (but still one of the cheapest in the UK market).

As others have said, you can also just buy ETF versions of most of their funds, which are regular shares traded on the stock market. In that case you're buying in USD.

As far as tax is concerned it's treated just like any other investment (and if you're tax resident in the UK and still have an ISA allowance you can just put it in there and negate the tax entirely).