Lao is closely related to Thai. Laotian generally uses the Thai alphabet for writing, and I believe that the phrase discussed here is the same in both languages, as well as in Isan. Like the related Khmer alphabet, this is an abugida (or syllabic alphabet) rather than a true alphabet.
In practice, you may still be understood without the proper tones for such a common phrase (especially given the contexts you would be most likely to use it in), but it is definitely better to try to achieve the proper tone when speaking.
ສະ = sa (with a low rising tone)
ບໃ = bay (with a low tone)
ຢ = y (with a low rising tone)
ດີ = dee (with a low tone)
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