In 1918 Valdemar registered for the draft in Fullerton, Vernon County, Louisiana (attached).

He was a chemist and listed nearest relative as Mrs. N. P. Christensen of Hornsyld, Denmark.
(This implies that he was not married, and that his father was deceased)

I can not find him in any U. S. censuses or other records.

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Hornsyld is in Nebsager parish, Vejle County:

Niels Peter Christensen and Maren are in the attached 1911 and 1916 Nebsager censuses, and in 1916 Antonius Christensen was also in the household.

But if N. P. died before Valdemar's 1918 registration, I don't find his death in Nebsager parish.