Thank you ycs for your valuable input and shoutout big thank you also for all you lowyat forumers who contributed too !
Before I went I  called JKJAV hotline 1800-88-8828 to check on opening/closing hours. I also checked IDCC facebook, but nearly didn't go because they had a gentle reminder that they accept only appointments, but they replied their FB posts walk in was also available (confusing). Glad to find out at this forum who confirmed, before me making long trip from KL.
I can confirm that IDCC is indeed having 1st/ walk in dose/ or appointment and 2nd dose for Sinovac. I am a local Malaysian and went as walk in for 1st dose yesterday. These below are some pointers that I saw:
IDCC Shah Alam, go to Car Park Level 4, follow signs HCO B (Sinovac), HCO A (AstraZeneca). Note: Disregard signs of Level 7 vaccine hall. Go and park directly at level 4 because that is the queue at the carpark. Disclaimer: I didn't try to go Level 7, but could have possibility saved me some time as the queues where very long at 11am, and i was glad to get in earlier. Also from the queue at Level 4 carpark, we go into and take escalators to Level 7 ( I didn't see another queue. So I am confident, unless proven wrong  ). The queue was long maybe because of the recent announcement of closure of all Klang Valley PPV starting 15 Sept. Be aware that this can change anytime ! KLCC closed down 1 day earlier than scheduled on 8th Sept according to FB page, but the JKJAV website said 9th Sept, and MBPJ Dewan Sivik is said to be 30th Sept on 8th Sept update (confusing)
There were 2 lines to queue at the car park HCO B, make sure you get into the correct line. One line for 1st dose, one line for 2nd dose, they go at different paces. Best ask and confirm with people who working/volunteering there. A guy behind me had Covid 20 days before and had his MySejahtera appointment cancelled, for his 2nd dose. He was in the line behind me. He was also a foreigner. But I cannot confirm whether walk in was for foreigner also. 
The people working/volunteering there was very efficient, there were people coming to our lines to make sure our MySejahtera was updated and refreshed. There were a lot of workers/volunteers there, I was very impressed with the standard of their professionalism and how they did their jobs, every step of the process was well thought of and well executed.
In short, always ask them if not sure of anything. And check your details before you leave. My vaccine card had an error and I had to walk back up to correct it.
Also, for those who like taking "I am vaccinated photos", the booth was on the left hand side of the EXIT, before the escalators. Once you go down, there is no turning back. Hope these info helps !
Also live music was great !  the lady busker yesterday morning sounded like ELLA