No, you're simply installing a certificate that serves to bolster the legitimacy of the WiFi access point you're connecting to (and encrypt connections between you and the router).
If you're talking about SSL decryption, of the kind done in your average corporate network, no. By accepting their certificate, you're not accepting a certificate authority-- you're simply accepting that the certificate will be used to encrypt the connection between you and the router. Unless you're using a VPN service or encrypted DNS, they would be able to see the hostnames you're visiting, but nothing more.
The security of a VPN service shouldn't be impacted by this at all. Again, you're not accepting any sort of certificate authority. That certificate can't be used for anything other than encrypting the connection between you and the router. OpenVPN profiles and such come with their own certificates, anwyay, if I recall correctly (tell me if I'm wrong here).
cheers
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