I am trying to run a function in bash to find out total VG size, VgFree

I am trying to run a function in bash to find out total VG size, VFree so that it helps to find the volume group which is lesser than 10G in size and send email alerts. I started to write few lines to find VG_SIZE. Once I come to use if statement to call every single value from a function to compare with certain value then I couldn't. Please help me.