I would suggest to use `notmuch config list` to get configuration
values in vim if possible

Reproduction example follows:

 /home/sh> cat .notmuch-config
 [new]
 tags=new;inbox;unread
 ignore=

 [search]
 exclude_tags=deleted;spam

 [maildir]
 synchronize_flags=false

 /home/sh> notmuch config list
 new.tags=new;inbox;unread
 new.ignore=
 search.exclude_tags=deleted;spam
 maildir.synchronize_flags=false
 database.path=/home/sh/mail
 user.name=Sergei Shilovsky
 user.primary_email=sshilovsky@gmail.com

 /home/sh> vim -c :NotMuch
 Error detected while processing function
<SNR>13_NotMuch..<SNR>13_folders..<SNR>
 13_new_buffer:
 line    6:
 TypeError: no implicit conversion of nil into String
 Error detected while processing function <SNR>13_NotMuch..<SNR>13_folders:
 line    2:
 NoMethodError: undefined method `query' for nil:NilClass

Having explicit database.path=/home/sh/mail in the configuration file
makes it work

-- 
С уважением,
Сергей Шиловский
Sergei Shilovsky