It's simple to require your clients to fill out certain questions on your questionnaires. When you're building your questionnaires (Settings > Questionnaires), just click the 'Required' checkbox under the Question Settings for a particular question. You can make all of your questions required or just a few. It's up to you. The Required checkbox is an option on all question types (except Instructional Paragraph, of course). Required questions will appear with an asterisk (*) in front of the question.
Then when you send a client a questionnaire they must fill out those questions that are required prior to clicking the Save and Submit button. If they do not fill out those questions, a "Please fill in all required questions" message will appear and those unanswered questions will appear in red. They will not be able to submit the form until all required fields have been filled in.
Note: When the question type is 'Vendor' and the client selects 'Other' in the drop down menu, only the Company Name or Primary Contact will be required in the sub-fields. The other fields such as phone number, email address, website, etc will be optional. This will occur whether or not the vendor question is required.