I've used CD4051 @ 5V VDD and control signals. If I use it @15V, datasheet says VIH should be at least 7V, VIL should be at most 4V. So in order to control the chip should I apply 7V or higher voltage to the control lines? Is that right?
Wrongly quoted. For Vdd=15 and Vee=Vss, Vih,min is 11V rather than 7V.
But do you actually operate the device with single supply?
Requirements are rather simple, control input uses Vss and Vdd supply and needs regular CMOS levels, Vil,max about 30% and Vih,min about 70% of logic supply voltage.
Consider that CD4051 is a CMOS logic device, not an analog switch with TTL control interface.