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In Lewis and Short, some vowels aren't marked. For example in this entry (chosen at random)
dŭŏ-dē-vīginti, card. num., eighteen, Plaut. Poen. 4, 2, 74 sq.; Cic. Ac. 2, 41, 128; id. Rep. 2, 22; Caes. B. G. 2, 5 fin.; id. B. C. 3, 71 al.; cf. also: annis mille centum et duobus de viginti fere, etc., Eutr. 10, 9.
Are the two last i both short or is their quantity unknown?
Many dictionaries, including L&S, are most concerned with vowel length for reasons of meter. And in meter, closed syllables (syllables ending with a consonant) are always heavy, regardless of the length of the vowel. This is traditionally called "long by position" (though "heavy by position" is a better term).
So when a syllable is heavy by position, L&S don't bother marking the length. They simply don't consider it important, since the expected use case for their dictionary is scanning poetry rather than speaking out loud.
Other vowels aren't marked because they're considered obvious. A second-declension masculine word will always have short -ŭs rather than long -ūs in the nominative singular, and long -ī rather than short -ĭ in the genitive singular, so L&S have decided not to mark those. And in particular, final -i in a plural noun or adjective is always assumed to be long unless otherwise specified (just as in locī). In vīgintī, it's not obvious at all, because this word is not in any way second-declension—it's indeclinable. But that's the reasoning behind it.
