VPN will be through Wifi only. If you need it when away from Wifi, you'll need an account (no contracts these days - it's usually pay by the month).
If you're looking for a VPN to keep your data from anyone's eyes, any of them that encrypt end to end will work - free or paid. Also, using https sites (as opposed to http sites) encrypt the data end to end - with no VPN.
If by "transportation hub" you mean airports, a lot of their Wifi connections are not free - so one SIM card to cover the entire trip might be cheaper. It depends on where you'll be. (In the US, Mint Mobile is about the cheapest - $15-$25/month, depending on the amount of data you need.)