Who designed for Vamo Sanderborg?  

The name is just Vamo. Sønderborg is the location.
There is no evidence that Henning Kjærnulf ever designed for Vamo. That style of seat frame with the front and rear stretchers set a bit below the plywood seat is not uncommon in Danish modern. I would not consider it at all sufficiently indicative of a designer to support a design attribution.
The backrest is the most emphatic feature by far. However it is also not a surprising backrest for Danish modern, so it would be hard to attribute it to a designer.
I would say the chair does not look like it would have been designed by Vodder & Borg, nor Torbjørn Afdal. And I would strongly hesitate to use a process of elimination to assume the next designer in the list is attributable for the chair. Danish companies often worked with lots of designers and records are scarce.
This chair is not in my 1957 Vamo catalog.
What does the Vamo mark look like?

I do understand that Sonderborg is the place name, but since some others may not, I put the whole thing.... Here's the mark:

I usually think of that mark as a bit later, and it has to post-date 1958 or so, because that black circle was a Danish Furniture Control mark. (See a similar Vamo mark below).
The fact that they are rosewood is also suggestive of an origin in the 60s as well.
Vamo was advertising the Afdal chairs in 1965, and I would not be very surprised if Vamo had started collaborating with other designers to freshen their product offering. This increases the likelihood that the list of 'usual suspects' does not contain the name of the designer of your chairs.

http://www.matslinder.no/2016/01/10/torbjorn-afdal-for-dansk-mobelprodus...