I'm not sure what type of product V-lights is, but my guess is that there is too much pigment build up in the hair. The gray coverage series in most color lines are usually double pigmented neutral. They also contain more ammonia to really drive all that extra pigment into the hair.
If you have been pulling the root formula through the ends as a refresher you may not get it to budge with anything. Those pigments are solid and they create a lot of color buildup.
The chance of buildup is so strong that I personally avoid even using them on the root. Unless the client has a history of being super resistant, I rarely use them for more than half of the formula. Most people don't even need them. The average gray client can usually achieve nice coverage with a regular neutral in the formula. But that's another story.
If having blonde highlights is really important to this client you might have to first do some sort of all over color remover to get rid of some of the excess pigment (like goldwell color remover, elimin8, or uncolor mixed with water) and start over completely on both the base and the highlights.
I would think this process would be too agressive for the desired result. Also, this type of process holds a delicate balance. I would not attempt it unless you have a long and succesful history with color correction.
The easiest answer would be to wait it out a few applications and try again. I would definitely try to get the client to let some of the color fade naturally. In the mean time, do not put anything on those ends! If the ends need some tone you can use a level 9 semipermanent, but thats it. You want them to be less solid than the root..
Take this experience as a lesson. Most cleints don't need to be refreshed with every retouch. Try to make a judgement call on when to refresh from now on so you won't end up in this situation again. Every time a client sits in your chair make a point of really looking at the ends in relation to the target color for the regrowth. If it looks right, then leave it alone. If it needs balancing, mix a second formula two shades lighter in the semi equivalent. If it's too light, mix the same color only in the regular neutral series.
As I said before, I never use that type of color on hair that isn't resistant gray. The ends aren't gray anymore so you don't need it. Same goes for refreshing ends with any color that has a heavily pigmented tone. Intense reds, violet or red-violet, any "brown" bases (like the wella /77 series), anything under a level 5. All heavy colors have to be refreshed with caution.
One of the targets for being a good colorist is to think about the long haul. Really think about what you might have to do down the road if the client wants to change.
Even if the client is 80, I always ask myself if I would be able to get her platinum in two visits. If I couldn't, then I rework it. You never know what they might ask for in the future, never back yourself into a corner just for the color they have today. There is always a way to achieve the desired result without ending up stuck down the road.
Of course, this is all just my guess of what happened. You may be doing everything perfectly and just have a weird situation, but I hope that my advice helped. Good luck and Happy New Year!