what you don't see is the very fine back spray dropping on the ground and washing down to the roots . the ivy needs pulling out the old fashioned way . what you can do is dilute the weed killer bye watering around the tree roots a lot and keeping your fingers crossed till next year .

What kind of weedkiller was it, the kind that is absorbed through the leaves (roundup or just glyphosate) or the kind that works through the soil and kills everything it contacts?

As Noseypotter says, even the lightest breeze will cause spray to drift and damage other things. If it was the first kind you may get away with it - I would wait until spring and see if it comes into leaf.

A temporary way of dealing with ivy is to simply cut the stems off at the base and leave the tops to wither and dry, when they are easy to peel off. If it is growing on your side of the fence you can then treat the roots permanently with SBK deep root killer, following the instructions on the pack.

ive got to say steragram in the case of the ivy I had too take down I cut threw the stems and it stayed alive and used sbk and yet it still lives . I also poured loads of salt on it but that only slows it down . I just keep on top of it now . you may if you look closely see new buds on your plum tree as I can see on my acre or scratch a little bark back to see if its still alive .

I do remember your long battle with the ivy Nosey, I think we were all feeling pretty involved with it by the time you finally got topside of it, but it was much bigger and more rampant than you could fit onto a fence! My suggestion works for smaller quantities (And I discovered that the dry stems make fantastic firelighters too!)
Anything's worth a try!

I totally agree sg just saying it doesn't always work .