Are you attempting to use an older version of Vensim?
Apple has dropped support for 32 bit applications. Versions of Vensim older than 8.0 are all 32 bit, so will never work on Catalina. All you can do is upgrade to the latest version of Vensim.
If you are using the latest version of Vensim, Apple has quarantined the software for some reason (Vensim is fully notarized by Apple so should not be flagged as malware). In this case, you can start a terminal window and try one of the following commands.
|Vensim DSS||sudo xattr -rd com.apple.quarantine /Applications/VensimDSS.app|
|Pro||sudo xattr -rd com.apple.quarantine /Applications/VensimPro.app|
|PLE+||sudo xattr -rd com.apple.quarantine /Applications/VensimPLP.app|
|PLP||sudo xattr -rd com.apple.quarantine /Applications/VensimPLE.app|
|Model reader||sudo xattr -rd com.apple.quarantine /Applications/VensimReader.app|