is 8GB enough for iOS dev and occasional VM?

According to Visual Studio's system requirements, it requires 1.5GB when in VM. Usually you will assign a max of 4GB on the windows VM which should be sufficient for a satisfactory operation of Windows inside the VM with the Visual Studio.

In this case there will be 4GB left for the hosting OS. Which usually is again adequate.

Also, Yosemite does a really good job in handling CPU and Ram, so in my opinion you should be ok.

My personal experience with a 2009 MBpro, 8GB Ram on Yosemite running windows on VM shows that there are very rare the occasions where I am out of RAM.

Sizing RAM and HD space is always been "get more than you think you need and all that you can afford." but realistically 8GB should work (like FFrewin indicates in excellent detail) but depends on your usage patterns. So it's really hard to answer.

"Mr Lincoln, how long should a Man's legs be?"

"Long enough to reach from his body to the ground"