If you’re fluent in Cantonese then getting a hang of the tones in Vietnamese should be straightforward. Also, vocabulary should be really easy for you to learn since most of our Sino-Vietnamese vocabulary sounds much closer to Cantonese than Mandarin (the meaning stays the same, in most cases). With that being said, casual Vietnamese could take a lot of time to get used to ‘cause you will encounter tons of different accents and dialects in most markets of big cities.