Do vowel tones bleed into other letters, including adjacent vowels?
I'm not exactly sure what you mean. There are different diacritical marks, some for a unique vowel sound and others for tones.
For example: ă, â, ê, ô, ơ, ư are distinct vowels.
Then there are the marks for the tones: sắc, huyền, ngã, hỏi, and nặng. The tones are for the whole word. Those marks can be used over every vowel: a, ă, â, e, ê, i, o, ô, ơ, u, ư, and y, but only one tone per word: muốn, huyền, nước, hỏi, cũa, etc.