Manneh, 22, is in the final year of his contract and if he re-signs with the Crew, Vancouver will also receive Columbus' natural first round MLS SuperDraft selection in the immediately following draft.
Both Tchani and Manneh had fallen out of favor with their clubs in recent months. The development of teenage sensation Alphonso Davies made Manneh more expendable for Vancouver, while Artur has been starting next to Wil Trapp in the center of the Crew midfield in recent matches.
Manneh has shown the potential to be one of the top attackers in MLS, but has struggled with consistency and injuries over his four-plus year career. Overall, he has 22 goals and 12 assists in 101 MLS appearances, 62 of which have been starts.
“We take pride in our identity as an attacking team, so we are excited to announce the addition of Kekuta Manneh, one of the most dynamic, young, offensive players in Major League Soccer and a new addition to the U.S. Soccer national team program," Columbus head coach Gregg Berhalter said in a team release.
Born in Gambia, Manneh recently obtained U.S. citizenship and was called into the national team's January camp, though he is still not yet eligible to play for Bruce Arena's team while awaiting paperwork with FIFA to go through.
Tchani, 27, has seven years of MLS experience, including five seasons with Columbus where he was a regular starter in central midfield.
"We have acquired an experienced, athletic, box-to-box player in Tony, who will help solidify our midfield," Whitecaps head coach Carl Robinson said in a release. "Tony is someone I enjoyed working with during my time in New York and I'm excited to have him join our group. In addition, the allocation money provides further opportunity to strengthen our roster and depending on what's next for Kekuta, there is also the potential of additional returns."
Tchani was born in Cameroon, but received his U.S. citizenship in 2013 and appeared in a friendly for the Americans in January 2016 against Iceland. The midfielder then swapped the U.S. for his home country, and is now cap-tied to Cameroon after playing in two 2017 Africa Cup of Nations qualifiers.