RCW 9.66.010 suggests that in "every place... [w]here vagrants resort... [e]very act unlawfully done and every omission to perform a duty, which act or omission [s]hall offend public decency" is an offense. However, my understanding is that the Washington state statute defining vagrancy was repealed in 1975. In this case, one would suggest the definition of vagrancy would default to that definition held at common law - the question is, what is that definition? Or is this clause simply invalid?
In Papachristou v. City of Jacksonville, SCOTUS found that a particular statute criminalizing being a vagrant was unconstitutionally vague. The court does not offer an alternative acceptable definition of "vagrant". Washington (and other states) dealt with this by repealing the law against being a vagrant, but left intact the law criminalizing "maintaining a public nuisance", which includes houses of prostitution and being a place "where vagrants resort". I have not located any subsequent ruling within Washington that indicates that this clause applied to "places" is enforced, and there is no jury instruction defining "vagrant". Instead, laws are written to prohibit specific acts (such as panhandling, sleeping on the sidewalk, prostitution).
Cornell characterizes a vagrant as "a person who roams from place to place without a permanent job, home, or material resources", with no supporting citations. This article offers a competing common law definition as "a person who was idle, refused to work although capable of doing so, and lived on the charity of others". In Johnson v. Florida, 202 So. 2d 852, the court cites F.S.A 856.02 (since repealed) defined vagrants as
Rogues and vagabonds, idle or dissolute persons who go about begging, common gamblers, persons who use juggling, or unlawful games or plays, common pipers and fiddlers, common drunkards, common night walkers, thieves, pilferers, traders in stolen property, lewd, wanton and lascivious persons, keepers of gambling places, common railers and brawlers, persons who neglect their calling or employment, or are without reasonably continuous employment or regular income and who have not sufficient property to sustain them, and misspend what they earn without providing for themselves or the support of their families, persons wandering or strolling around from place to place without any lawful purpose or object, habitual loafers, idle and disorderly persons, persons neglecting all lawful business and habitually spending their time by frequenting houses of ill fame, gaming houses or tippling shops, persons able to work but habitually living upon the earnings of their wives or minor children, and all able bodied male persons over the age of eighteen years who are without means of support and remain in idleness, shall be deemed vagrants
Common law definitions arise from appellate cases that approvingly restate and unify prior court and statutory definitions (for example "negligent"). The dearth of approving rulings since Papachristou indicates that the concept itself is in jeopardy. A test could arise if a municipality decided to outlaw homeless encampments, however, such a law would be partially preempted by a state law allowing a religious organization to arrange a homeless encampment (plainly a violation of the First Amendment, but it would mean that a generic non-religious organization might be found guilty of violating a local analog of the "place where vagrants resort" law).
Hence I conclude that the courts would also find that clause to be at least void for vagueness.