Adding LFO Modulation to VCA in synthesizer
The crux of your problem seems to be that you want to multiply the LFO signal with the A/R signal, and to do this multiplication requires circuitry that is effectively a second VCA.

One way of making a VCA is by making use of pulse-width modulation technique as Andy aka suggests. The classical 'analog synth' method of doing so is by altering the bias current on a transistor differential pair -- usually within a transconductance op-amp. See pages 8-10 or thereabouts of the LM13700 OTA datasheet.

The above talk of multiplication and signal VCAs assumes that the input to your existing VCA is linear (which I believe is the usual case). If the input is in fact logarithmic, you simply need to sum the A/R signal and LFO signal (e.g. using an op-amp mixer) before sending it to the VCA.