Don't remove VCC to a chip without removing voltages to all other signals.

If you feed 5V to other pins, they will normally supply voltage to the VCC line. Many chips behaves as if they have a diode from the different I/O signals to VCC, so you get a diode voltage drop.

Some chips can handle a lot of abuse - much thanks to these body diodes. Some chips goes up in smoke for almost no abuse. When in doubt about availability of power to all signals, it can often be good to have a series resistor on the signal lines just to make sure that they can't be used to feed to much power into the chip the wrong way. In the case of 8051 chips, the weak pull-up in the processor will limit the amount of current sent into the ADC.

Edit - forgot to ask you: Have you verified that the /CS and /RD signals gets activated when you try to read from the ADC0848? When not addressed, the ADC will keep the data signals in tristate and your processor will then just read its own pull-up resistors.