Ok, I've got TWO options for you to choose from:
Option 1: Config-Register 0x2102
Configuration Register value of 0x2101 is, by default, the setting when the appliance or supervisor is shipped out. The last octet of "1" basically tells the appliance to IGNORE the boot variable string and boot the first valid IOS (from top to bottom) found in the bootflash.
The easiest and fastest way to do an IOS upgrade on a VSS pair is very simple.
1. Copy the IOS from the TFTP server to the active unit using the command "copy tftp://<TFTP IP address>/IOS_filename.bin bootflash:";
2. Copy the IOS from the TFTP server to the standby unit using the command "copy tftp://<TFTP IP address>/IOS_filename.bin slavebootflash:";
3. Remove the old boot variable string: no boot system flash
4. Change to the new boot variable string: boot system flash bootflash:IOS_filename.bin
5. Change the config-registry value to 0x2102: config-registry 0x2102
6. Save the config.
7. Reboot.
Now be aware that, like any 4500 supervisor card and 6500 supervisor card, if you change the config-registry to 0x2102 and you DO NOT HAVE a boot variable string (or a valid one), the your switch will reboot into ROMmon. So please take good care in entering the right boot variable string.
Option 2: Config-Register 0x2101
Now, if you do not want to change the config-registry value to 0x2102, this means that you would like config-registry value of 0x2101, there's another option around this.
And the process is almost similar.
1. Copy the IOS from the TFTP server to the active unit using the command "copy tftp://<TFTP IP address>/IOS_filename.bin bootflash:";
2. Copy the IOS from the TFTP server to the standby unit using the command "copy tftp://<TFTP IP address>/IOS_filename.bin slavebootflash:";
3. RENAME the old IOS in both units:
Active: rename bootflash:OLD_IOS_filename.bin bootflash:OLD_IOS_filename.bin.BAK
Standby: rename slavebootflash:OLD_IOS_filename.bin slavebootflash:OLD_IOS_filename.bin.BAK
4. Save the config.
5. Reboot.
You need to rename the old IOS because if you don't then there's a chance that one of the VSS pairs will boot the OLD IOS if the file is found to be above the new IOS file.
Hope this helps.
PS: I've stopped attempting to upgrade via ISSU because it's never worked out for me. Either one of the two options are "workarounds" when ISSU doesn't work.
The type of the pre- and post-upgrade images must match precisely. Identical. ISSU is not supported from a Universal_lite to a Universal image, or vice versa. ISSU is also not supported from a k9 image to a non-k9 image, or vice versa. 
