Too bad about the grounded "source" connection of your "high voltage vacuum FET", you dont wanna add much resistance in there. Probably best to use 1 ohm and amplify the result.
Use an op amp that goes to the negative rail and you can run it off a single supply +5V. LM324 works, other choices may have lower offset error, etc., but remeber you can trim offset in software
I'd say about 1k and 50k would be good resistor choices for a +5v Arduino, that gives +5v for 100 mA.
Wouldnt hurt to add a series resistor for Vi maybe 10k just in case the input gets connected to anything bad.