Just be aware that if you do this some ICOs require to you sign a disclaimer or acceptance of some terms and conditions explicitly stating you are not a US Citizen or resident. Some even threaten to hand over your info "to the authorities" if you are found to have violated this.
Even if you use a VPN remember that you also leave a trail because of any email you use, where your cryptocurrency to pay into the ICO came from, any subsequent interactions with the ICO site. A lot of people in the US are going to do something like transfer some BTC or ETH to a local wallet, then contribute that to the ICO (because you often need a non-exchange source of funds otherwise your ICO coins may end up owned by Coinbase, Poloniex etc.).
That kind of funding pattern leaves and easy trail of funds to the ICO and back to Coinbase etc. which investigators good, at some future dates pin on you. I personally think it's unlikely thanks to the current Trump dministration's aims to defund or incapacitate watchdogs that could do it - like the SEC - but you never know.
If you are paranoid or investing huge amounts you could not afford to lose then you might want to a) think about whether investing in an overseas ICO is for you, b) think about more robust anonymity policy.
RE: How To Invest in ICOs From the United States