According to the Proctor & Gamble msds (P&G makes Zest) which you can find here: http://www.pg.com/content/pdf/01_about_pg/msds/beauty_care/personal_cleansing/zest/Zest_Clean_Moisture_Bar_Soap.pdf;jsessionid=JDRU055D2Q0FBQFIASJXKYWAVACJE3MK
If Zest is accidentally ingested, it is not expected to cause gastrointestinal irritation. Accidental ingestion of undiluted product may cause mild gastrointestinal irritation with nausea, vomiting, and diarrhea.
It also says that you should not induce vomiting and that you should treat the animal/person who ate the soap symptomatically...meaning, if they end up with diarrhea, you treat for diarrhea.
For tonight, I'd just keep an eye on him and if the vomiting doesn't resolve itself by morning, I'd call your vet.
I hope this helps!