I was thinking about this same thing last year and I did a bit of research on that. The number of airframes available is quite limited (I think just 24 planes), I don't think its worth all the logistical hassle and the planes are not going to be able to match the chinese J-10 A / B anyway. Colombia has been buying quite a few of those and that makes sense, they are not facing a high tech enemy.
After checking on that, I fantasized about getting upgraded mirage 2000's and I do feel good about that, much more capable plane. The Kfir is a copy and upgrade of the Mirage 5, quite outdated at this point, but still, if the quantity were to be good, and with a good AESA radar, etc and assuming the plane would be mostly for ground / naval attack and is fitted with good anti ship missiles, well, it could be ok, but in the air defense role I think not.
One other issue could also be that the Israelis have not interfaced the Kfir avionics with the type of anti ship missiles that Vietnam has, so that would need to be implemented, I'm not sure how easy that would be.
I think the Tejas MK2 would be a better deal and is also very cheap (whenever it becomes available).