Hello There,
I been working on vtiger platform ever since 5.x versions starting from 2008 onwards. I can develop modules, customize existing module, integrate third party API with vtiger, write custom CRON and workflow etc. I have done many complex customization in vtiger.
I have customized Mail Manager to have multiple email ids i.e one user can use multiple email id in mail manager.
Let me know what kind of customization you are looking for.
Looking forward to hear from you
Regards
Ajay