The 1892 RLDS edition accidentally omitted the word very here in Mosiah 1:9. The subsequent RLDS edition (1908) restored it. Several references to soon going out of this life occur elsewhere in the text:
In none of these does the adverb very appear with soon, which could explain why the 1892 edition accidentally lost the very in Mosiah 1:9 (especially since the first three of them are found close-by, in the last part of the small plates of Nephi).
There is, of course, nothing wrong with very soon occurring in the text, as it does in two other passages:
Summary: Maintain in Mosiah 1:9 the reading of the earliest text, very soon.