Copyright is complex; in order to answer properly, I need to know which work it is.
Most works by w:Vajiravudh will be PD in the US. I've never looked into Thai copyright law, but I will if it is necessary to give you an opinion on the work that you have in mind. John Vandenberg (chat) 02:28, 21 March 2008 (UTC)
Is it this book: 
Any of the works of Author:William Le Queux published before 1923 are PD, and w:Vajiravudh's translation will also be public domain if it first was published before 1923. To figure out copyright, we need to know the publication history of the Vajiravudh translation. Does the book say when the work was first translated? John Vandenberg (chat) 12:25, 21 March 2008 (UTC)
Does the book mention when & where he published the translation?
Does your book mention an editor; i.e. a modern person who helped compile the work?
That sounds like it may have been "unpublished" at the time of his death, and maybe first published in the edition in your possession. To be considered "published", copies for sale need to be produced. There are special copyright rules for unpublished works. John Vandenberg (chat) 13:59, 21 March 2008 (UTC)
The key question is whether this work has been sold for profit previous; if not, then this is the first "published" edition even though private printings may have previously been produced. If it was published prior to 1923, then it is definitely PD. If it was between 1923 and your edition, we need to know which year it was first published. Here is a list of books held written by him - perhaps you can find an earlier edition in there. John Vandenberg (chat) 15:49, 21 March 2008 (UTC)