Hi anton8359,
I would suggest creating a custom record for your certifications which could then be added to a subtab on employee and vendor records:
1) You can start by creating a custom list for your certifications
2) Create a customer record to hold the certification information (i.e. Certifications)
3) On your custom record, create list/record type fields sourcing your certification list, employees, vendors and date fields to hold the start date and expiry date
4) Create a saved search on your custom record
5) Create a subtab (applied to vendors and employee records) and a sublist to hold the saved search
On the vendor and employee records, the sublist will display the list of certifications related to the current vendor/employee
Hope this helps !