Driving from Austria to Croatia you cross Slovenia in about two hours. Slovenian off-motorway fuel prices are regulated and usually the cheapest on the whole route. Refuel here, not at motorway service areas, where prices are not regulated.
Yes. Regulated off-motorway prices in Slovenia are usually below Austrian and Croatian prices for petrol 95, and diesel is comparable or cheaper.
Yes. An e-vignette is required for Slovenian motorways and expressways; buy it online or at filling stations before the border.
On the Karavanke → Obrežje corridor it is around Novo mesto (take the exit); heading for Gruškovje it is Ptuj or Podlehnik.