Will my home network become a target for hackers if I run a bitcoin node or run bitcoin mining from devices in my home network?
Mining Bitcoin? Unlikely since only pool know your IP.
Running Bitcoin node? More likely since your IP will be known to other node and appear on crawler website such as
https://bitnodes.io/.
Would a VPN help or is it better to run bitcoin node / mining in a completely different network?
Both option could work, but i don't know how much security risk could be decreased. Besides, you need to trust VPN provider when we're talking about privacy.