Yes, it could be a disadvantage. What is boils down to is how much you trust the VPN provider.
For most secure protocols, using a VPN will be just as secure because your communications are encrypted by the protocol. If there was a MITM at the other end of the VPN connection they would not be able to do much (apart from a side channel attack, which are usually pretty useless in isolation). Of course, this is assuming the protocols and software are secure, and cannot be not affected by the FREAK attack or other downgrade attacks.
However, the web is different. The main issue is that the Same Origin Policy does not designate a different origin for plain vs encrypted where cookies are concerned. A cookie set on
http://example.com can be read by
https://example.com. If there are any cookie handling vulnerabilities on the site then the "secure" connection could be compromised. The Secure Flag does not help here - this only prevents a plain HTTP connection from reading a cookie set over HTTPS, not the other way round. An example could be cookie poisoning like session fixation, or if there's an XSS vulnerability based on a cookie value that was assumed to only have been set via HTTPS. These are really vulnerabilities on the sites themselves, however using an untrusted connection allows them to be exploited.
So if there is any doubt about the trust of your VPN provider, then disable plain HTTP from your browser and use the internet over HTTPS only. You can do this by setting an invalid proxy server for plain HTTP (e.g.
Of course, you should make sure you are using a secure protocol for your VPN connection too (e.g. not MS PPTP). Also, make sure you use iptables/Windows Firewall properly to prevent any incoming connections to your machine whilst connected to the VPN.