Every word in Vietnamese has its own meaning (there are still some exceptions); however, when you combine words together (usually two words or a compound word), they will have a new meaning; sometimes the new meaning is similar to the single word in the compound, sometimes it is totally different from each other. For example, "khoa" means department, "học" means study, "khoa học" means science. The better way to learn Vietnamese vocabulary is to learn both the single words and their corresponding compound word. But you can also try to guess the meaning of a compound word based on its each single word.
Vietnamese has six tones (e.g. ba, bà, bá, bả, bã, bạ). Each tone results in a different word with a different meaning. For instance, "ba" means dad or number three, "bà" means grandma or the title Mrs. The fourth and fifth tones, "dấu hỏi" in "bả" and "dấu ngã" in "bã", have similar pronunciation but different meaning. For example, "đả" means hit, "đã" means was/were (use to emphasize something happened in the past). Southern Vietnamese accent usually uses the fourth tone to pronounce for the fifth tone also.
Vietnamese has three different types of word: pure Vietnamese words (e.g. "ba"), Chinese-Vietnamese words (e.g. "mã"), and foreign-borrowed words (e.g. "xích lô"). When using an adjective to describe a noun, the noun usually precedes the adjective like in Spanish (e.g. "camisa bonita"= "áo đẹp"); but if they are Chinese-Vietnamese words, the adjective comes before the noun like in English (e.g. "white horse" = "bạch mã"). To determine the word origin, you need to check the dictionary. Or as mentioned above, you should study both single and compound words.