The country has a long coastline, varied, complex climatic zones and generally a fair amount of rain.
The best beaches are mostly in the southern half, and that's best Dec- May [dry, but hot and humid]. However, in that area it doesn't usually rain for long periods at any time, only showers, unless an infrequent typhoon hits the coast, so the beaches are basically OK all year round.
The worst season is probably July -Oct [humidity, occasional rains, typhoons, and of course July-Aug is always a busy holiday period].
Oct - Dec has the best weather over the whole country for sightseeing, with clear skies and temperatures in the mid 20ÿCs.
Plan ahead if you wish to travel during the Tet New Year festival [late January to early February] - it can be a problem getting accommodation and transport.