In theory, this would work assuming two factors:
1> Your Fiddler-running server is accessible via your VPN, and
2> Your IPhone is willing to respect proxy settings when a VPN is active.
You should be able to try this out using the standard steps for using Fiddler with an iOS device. First, ensure that, with your VPN active, your browser can reach http://fiddlerserverip:8888/
at all. Then try setting your proxy settings and see if they're respected when the VPN is active.
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