Delta Chat supports importing vCards already.
There is a scheme called MeCard that contains a single vCard: MeCard (QR code) - Wikipedia
It should be easy to implement it at least to add unencrypted contacts. For encrypted contacts the QR code may be large because of the key inside and nobody generates such QR codes anyway.
I just checked the sizes of couple of DC generated vcards (for encrypted contacts, with keys).
If there is no picture, the size is usually around 700 bytes.
E.g. libqrencode manual says the capacity of QR Code is up to 4000 characters.
With embedded picture vcard size card can go up to 60+ kbytes (or even higher - I just tested small amount, and not looked ad DC app code).
IMHO it’s better to include the code and downscale the picture (and maybe encode it to PNG with reduced color pale) to fit into 4000 chars.
May I ask, what are the advantages over current vcard format?
An open standard for contact QR codes would be nice. I am not familiar with MeCards. They look decent except for the name field; of all places for software to make cultural assumptions about name ordering, Japan?
I think opgpcard produces QR codes of VCards containing key fingerprints and a keyserver link; you can also just omit fields. This makes them resonably compact – I’ve got one at 255 bytes – but the dependency on the server is undesirable.