You should look in the compiler documentation for an "interrupt" keyword. This keyword will allow you to select which vector to use and more importantly tell the compiler to use an RETI instruction to return rather than the usual retrurn instruction.
This is one of those "extensions" to the C language whose implementation is definitely non-standard. Whadya gonna do! -- Eh? Fageddaboudit!
It has been a while since I worked with Keil tools. I took a look at the example suggested by Nanovate and see the "interrupt 0" qualifier on the ISR. I assume that the file REGS52.h maps this ISR to location 0013h as mentioned by Abby. A good source of reference information is the sample code available on the Keil and Atmel websites but one has to be aware of tool (compiler/assembler) specific twists. Regards.