Hi @ddeveloper,
I worked through this process just a couple of months ago (and I am not a developer 
) and managed to successfully migrate a self-hosted Vanilla forum to self-hosted Discourse. One thing that was key for me, was making sure when doing the data export with Vanilla Porter, to select “Vanilla 2” as the Source Forum Type in the first drop-down menu.
I used the Vanilla Porter 2.6 version available for download as a zip file here: Vanilla Porter 2.6 RC1 — Vanilla Forums instead of the 2.5 version linked in Migrate a Vanilla forum to Discourse.
If I remember correctly, I didn’t hit the UTF-8 error again when using the newer Vanilla Porter script and the “Vanilla 2” forum type.
If those two suggestions don’t make a difference for your import, please provide a few details about the steps you’ve taken so far and exactly what you’re seeing. Sometimes there are slight variations to “same error” that can make a big difference when troubleshooting.