VGA help

I am trying to build a single-channel transmitter of a medical system and later extend to multi-channel. I tried to use vector modulator and VGA to make it work. But the problem is the control of the VGA. I am not familiar with the digital interface (if I am using a digital variable gain amp). The freq is 200MHz. Thanks in advance. I want to arbitrarily control amplitude and phase in each channel. Any suggestions?

For the digital VGA control, you need specify the step size for each digital control, for example 2 dB/bit. Do you have A/D convertor in your system? How about AGC?