Should vamps protect some slayers? [spoiler]

As I mentioned in another thread, I am new to BTVS. I am just coming up to
the 
end of season 2. I am thinking back to the episode with the other slayer, 
Kendra. She turned up because Buffy had died, very briefly, earlier on.
This 
implies that when one slayer dies, another will soon replace her. From the

vamps' point of view then, there's no point in killing a slayer unless
that 
slayer is a particularly strong one like Buffy, since the replacement is
most 
likely not going to be as strong. Conversely, if the current slayer is a 
particularly weak one, wouldn't it be in the vamps' interests to protect
her, 
since if they kill her the next one is likely to be stronger?

Also, there seems to be a contradiction between having a new slayer as
soon as 
one dies and one slayer per "generation", unless "generation" is just the 
average time they last rather than a rule set down by the powers that be.
If 
there's really only one per generation then vampires should try to kill
them all 
as soon as possible so they can have up to about 20 years slayer free.