Depends on where in Vietnam you are thinking about in terms of north and south, not mountains or beach.
The southern and central parts of Vietnam will be quite hot in April (hottest time of year, avg temps in the mid 30s), while the northern reaches around Hanoi are still cooler (avg temps in the low 20s, their hottest time is June - Aug). April is still dry season in all areas.
Bali doesn't change a whole lot in terms of temperature between summer and winter (28 to 31 similar to Singapore), mostly in terms of how long it rains (dry season has short showers, rainy season has long downpours).
So in the long run, not a huge difference between them weather wise.