To get the best out of an addon nv is a combination of
nv unit,
scope,
ir illumination.
for the money you are looking at a digital unit and they can be more fussy as to scope. Unit wise you are looking at the archer arrow, which is in the process of being released, Marky 610 has developed the nite tek, there is also the nitesite.there may be others but they don't spring to mind, some one will add to the list I'm sure.
Illumination is laser, firefly, n1000 etc, these are about the size of a pen and the battery will last for about 60 hours. But because the glass is so small that any marks can show up and they often show concentric circles. Therefore the image isn't as "clean" as it could be. And you will need to clean the lens. Price ranges from £100 to £300 ish. Or ir torch such as the nightmaster, it is bulkier, the batteries don't last as long but it gives a very clean image, also some of the lads on the nightvision forum are now producing led ir and they are very cheap.
Scope. I don't know anyone using a vortex so I am inclined to thin that max range for you will be about 150 yards. The more you up the zoom. The more the image will dim.
with the right combination and the right conditions you will see out to 300 yards. But just because you see a pair of eyes in the distance doesn't mean they are a fox. Unless you can positively id the target body shape don't take the shot. There are too many horror stories. When stalkers talk about backstops and safety when out after deer well this is doublely important at night because you have such a limited field of view.
I may be teaching people to suck eggs here but it is so important, you must 100% be able to identify the body of the target otherwise you are going to shoot the wrong thing. I nearly shot a little owl a couple of times because I could see the eye shine on the ground. It was only that I held off because I knew it was a rabbit but I couldn't see the body then realised it wasn't a bunny.
SO JUST TO BANG ON ABOUT IT. SAFETY SAFETY SAFETY.
there is a lot of experimentation with nv to get the best combination. Some people have a hawk and they say it will do what they need, which is fine, but there is better that will see farther.
your other option is to go to the nightvisionforumuk and as the paper man says READ ALL ABOUT IT.
If you can solder you can have a go at making your own.
a point to note. I thought nv was not legal over in Ireland, or did I dream that?
i use to shoot out Bandon way, ohhhhhhh a few years ago now. We weren't allowed moderators then and a 22.250 was the largest calibre you could own and was used for deer. Shows how long ago that was.