Hi Ivan,
you have multiple options. You can use a single table with all attributes across all your different company types and then use different data structures to "drill down" to certain types.
You can also create different tables for each type eg
Company entity with all common attributes
Customer entity with specific customer attributes. Since customer is an extension of the Company table you set its Id attribute to Company Identifier.
What to take can depend on the amount of different attributes and your personal preference. One other thing to consider: If you are using Aggregates to query data it is better to create a table by type approach to minimize data retrieval. Advanced SQL gives you an alternative but maybe that is a little to tough to start with.
Another thing to consider is the licensing model. OutSystems switched backed to an AO based model for new contracts. An entity costs you 1 application object each. Having a inheritance by table model for Customer and Vendor would take 3 AOs. One for the common Company entity, one for customer and one for vendor.
Hope that helps,
Stefan