Add Vapi startup program

[Aspct3434]

Summary

• add Vapi as a new vendor
• add exactly one startup-specific offer with 90,000 free usage minutes, early product access, community membership, and technical support
• model the published company-age, funding, voice-LLM, and current YC-backed eligibility requirements
• cite both the program page and current application form

First-party sources

• https://vapi.ai/blog/vapi-ai-startups
• https://form.typeform.com/to/jmgDdUyW

Reservation

• Closes Missing vendor: Vapi Startups Program #152

Validation

• Local YAML/reference checks: 1 vendor, 1 program, 1 offer, 3 unique sources, 5 eligibility criteria
• git diff --check
• exactly one changed data file: vendors/va/vapi.yaml
• commits include DCO Signed-off-by lines