Depends what damage has been done really. If they collapse then they're permafucked as far as I know. Mostly they just hide deep down and don't come out to play anymore. Someone with more accurate medical knowledge could no doubt correct me and explain better but, from my perspective, the ones that start off big and healthy seen to shrivel and die, the slack is taken up by smaller (but still decent enough sized) veins who in turn get battered, shrivel, die and the slack is taken up by even smaller veins... and so on.
Obviously no limbs have dropped off yet so blood is still circulating but I have more or less permanent pins and needles (

) in the extremeties, feet in particular but also hands are permanently cold too. Circulation still circulates but barely enough to keep stuff working, I'd surmise. I always had atrocious veins so was never a good candidate for longterm IV use of owt but I have no visible veins left in arms or legs anymore. All there is to see - even with a tourniquet - are small and very wiggly capillary-size veins. There are still larger ones deep down but are all heavily scarred now which makes hitting them not only very difficult but also unbelievably painful. Dunno why (thought scar tissue didn't hurt?) but the pain is too extreme to put a needle anywhere near 'em these days. If I ever need to have blood tests done it takes the doc/nurse/phlebotomist literally hours of attempts. Last time was about two years back, took over two hours and she got maybe 1ml of blood in the sample and about a pint dribbled and dripped anywhere but the sample pot
Have heard that exercise is good to beef up blood vessels and really should make the effort... Tis exercise though innit 8)
Never heard of gingko in relation to vein health - may have to look into that. Especially if it doesn't require exercise to have an effect :D