Merging on Vendor releases

[Fabio Fracassi]

Hi,

I am fairly new to CVS, and need to do a not so trivial task with it. I've 
read all the Documentation I found (Namley the cederquist Online Manual, the 
FAQ, and a fair amount of the Mailinglist Archive), and tested some things, 
but am still a bit lost.

Here is what I want to do:
I have a CVS repository, with a initial vendor release import. From this base 
there were quite some local modifications to the code, which I like to port 
to the new release of the vendor, one at a time (We have several tags in our 
local repository).
Using CVS's import feature leaves me with a huge amount of conflicting merges 
(the vendor project is quite large) and as far as I understand would commit 
all our modifications at once.(Which would make the resulting code quite 
undebuggable) What I like to do is porting one local change at a time e.g. I 
have the following tags

VendorRelease
basicChanges
extension1
extension2
product

I'd  like to take the new Vendor version and first apply the basicChanges, 
then test and debug the whole mess, and then go on with extension1 and so on.

Now my question is this possible in cvs (I guess so)? If so how do I do it 
(From what I figured the solution lies somwhere in branching and diffing)?
Are there any tools which could assist me?

TIA

Fabio Fracassi