I often have to do this myself (and I'm a vendor manager) because of retirement or a severing of a relationship with an abstractor, OR because the client has done so.  Frankly, I hate doing that because it puts me in a bad light.  And if you ever suspect you're getting the updates (because you're cheaper) but not the original search (because someone ELSE is cheaper), I'd refuse.  No ethical VM is going to do that.

That being said, as long as I have a commitment or the complete search from a previous abstractor, I'll do the work (both as an abstractor and as a VM).  I have one abstractor who puts in a special heavy duty disclaimer and I certainly have no issues with that.  She states that she did not do the original search and is responsible only for X date to the present effective date.  It's just an added layer of protection for everyone.  I know she has a line in there to the effect ..... "...not responsible for any errors or omissions on the original search."