I hate deciphering artist monograms and signatures but I don’t see the first character as an ‘X’. It could be a very flamboyant ‘H’ or something else entirely.
For sure the scene is not Vietnam and I would think European. The hats worn by two of the figures are very Western. I take the ‘51’ to be the date of the work (1951).
To the left of centre there’s a street sign (possibly on a black and white striped pole) with white lettering on a black background and perhaps a right-pointing arrow. Local traffic road signs usually employ black text on white and the only country I know of that uses white on black is Finland.
To the left of that, there’s a vertically-reading sign on a building that says “…LBECH” and of the kind you might expect to see on a department store, hotel, cinema, theatre or the like. Whatever precedes the visible letters must be fairly short, and I wondered if it might read “HOTELBECH” in full. Although not common in Finland, ‘Bech’ is a traditional Nordic surname, derived from the word for “stream”.
None of that brings me closer to the artist, except in narrowing down a possible date and location.