vst~ is treated as a graphical object : when you crate a subpatch and make it "graph on parent", vst~ is treated as an object having a GUI.
 
Following is a demonstration of the problem.
 
tom f
 
#N canvas 0 0 454 304 10;
#N canvas 147 317 524 316 vst_test 0;
#X obj 458 285 vst~ 1 1;
#X floatatom 14 14 5 0 0 0 - - -;
#X msg 73 91 param 0 \$1;
#X connect 1 0 2 0;
#X connect 2 0 0 0;
#X coords 0 0 1 1 100 70 1;
#X restore 119 29 pd vst_test;