Strikes me as a bit strange that one of the prime deciding factors upon which the decision whether or not to contact a new race is if they possess a particular type of propulsion. After all, it is merely a technological achievement, and says nothing about the species as a whole.
There's a certain level of belief among Trek fans that Vulcans were visiting neighboring star systems centuries prior to their independently developing warp propulsion. And that the Romulans set forth on their grand colonization effort in STL ships, not FTL.
An analogy might be if nations on Earth could not join the UN unless they had independently invented the steam engine.
Instead, whether a world would be invited to join the Federation (or have first contact) should more properly be based upon the social, cultural and intellectual development of the species. While not perfect examples
, the Arab golden Age (8th - 13th centuries). Europe at the towards the end of the age of enlightenment (18th century), similar time periods in India and China. While lacking the technological "toy" of a warp drive, some cultures might be determined in the judgment of the Federation to be ready to join a interstellar community of sapient beings.
A relatively "primitive" alien culture could hypothetically possess abundant wisdom and awareness, attributes that conceiveable could be lost by waiting a millennium for the engineering skills necessary to build a warp engine. A race residing upon a planet with scad natural resources could create a society of philosophical knowledgeable citizens that could improve and change the Federation immeasurable with their inclusion.
After contact the nearby Federation member worlds could simply provide warp "bus service" to these people, if they wanted it.
Returning to the Vulcans, how does the Federation handle a species who doesn't need a warp drive for star travel, will Federation members continue to refuse contact when this species drops into orbit around their homeworld? At some point it will become kind of ridiculous.
What about a species that again lacks warp drive, but does possess warp communications? Would they be allowed to connect to the Federations "internet?" But just not experience physical contact.