Pfff, I just sorted out all my contacts. I built up 485 contacts over the years in Exchange. Some of them merged with other contact data on N900 and N9, like photo or address, maybe Skype, LinkedIn, I don't know. But the normal sync with Exchange didn't bring them to the Jolla. Then a sync over bluetooth from the N9, no joy either. In the end I exported all contacts on the N9 into vcf cards, browsed through the Jolla contacts to find out the missing ones. Painful, as I found no way to see a long list of names. The grouping by initial makes you go up again after finishing A to click the B etc. Anyway, I found 137 names missing. Merged the vcf files into one file. Sent it to the Jolla, no joy. In Excel I threw out all the lines I mistrusted, leaving plain and simple vcf cards with 1 phone number and 1 e-mail address. Pasted those into a new file, sent it to the Jolla by bluetooth and it worked! There may be an easier way, but I did it!
I had a similar experience. Couldn't import a single large vcf containing about 166 contacts (with avatars etc.), but could import 26 separate files if I joined all the a*.vcf >a.vcf, b*.vcf > b.vcf etc. Importing the 26 small vcf files went like clockwork. No idea why the large file wouldn't import considering it contained exactly the same data. My method for transferring the vcf files was to email them to myself and open the email attachment on the Jolla. I've now sent my large vcf file to w00t so hopefully Jolla can work out what the problem is and fix it because it's rather frustrating.
cat *vcf | sed 's/VCARDBEGIN/VCARD\r\nBEGIN/g' | cat >> testall.vcf
One part of my fix to vcardconverter will simply be preprocessing the input vcf and looking for lines which match the "END:VCARDBEGIN:VCARD" form, and changing to two lines: "END:VCARD"\r\n"BEGIN:VCARD". but as you say, there's still an outstanding bug there somewhere.