It seems to be common knowledge that a VPN has the potential to protect data that is being transferred through an insecure network. Does a VPN also protect a secure network/router from malicious website data being sent to a device using that network with a VPN? I am new to this site so please forgive any mistakes or issues and just let me know. Thanks
A VPN provides an encrypted tunnel between an initial device A and a server B where the data is decrypted and processed, for example by forwarding onto a requested server C. Responses from C are sent back to B, encrypted, and sent to A. This means a man in the middle, M, can't determine what A is sending to C in a scenario where all of A's traffic goes through M.
HTTPS does something similar - A could talk to C directly, and M would only see encrypted traffic. If C only serves data via an unencrypted connection however, such as HTTP, M would see all the raw data. This latter scenario is what VPNs are useful for.
In any scenario though, B does not (ordinarily) do anything to the content sent back to A. In this sense it's 'dumb'. If the connection between A and C is ordinarily not encrypted, then B could potentially perform packet inspection and check for known exploit code etc., but in practice this is impractical most of the time.
what you are talking about is called "reverse tunnelling". It does work (sort of) and used to be used to transfer traffic between low security networks across a higher security bearer. The basic concept being that the payload traffic is encapsulated (by a device to the same security standard as the higher security bearer) and then broken out at the other boundary. As the traffic is 'random junk' while it is transiting the high security network, it cannot impact any device that processes it. This is still used by some military networks.
However, there are detailed attacks against reverse tunnelling which are different to the attacks against VPNs - the crypto boxes used by the couple of military networks that I am aware took a long time to get right. Coupled with the wide availability of low trust networks nowadays (ie the internet) the investment in these techniques has been limited to specialist applications.