LPC2148 UART IRQ ISRs are not getting invoked

Below is my code where I need to do some business logic processing when a character / data at 2 UARTs arrives. But I am able to write dummy text to UARTs but not receive it via UARTs ISR:

#include <lpc214x.h>

#define Fosc            12000000
#define Fcclk           (Fosc * 5)
#define Fcco            (Fcclk * 4)
#define Fpclk           (Fcclk / 4) * 1

#define  UART_BPS       9600


void delay(int a)
{
        int j,i,k;
        for(k=0x00;k<1000;k++)
        {
                for(j=0x00;j<a;j++);
                for (i=0x00;i<a;i++);
        }
}

void usart0_string(unsigned char *data)
{
        while(*data)                                                    // transmit only
        {
                U0THR = *data;
                while(!(U0LSR & 0X20));
                data++;
                delay(15);
        }
}

void usart0_char(unsigned char temp)
{
          U0THR = temp;
                while(!(U0LSR & 0X20));
                delay(15);
}

void UART0_RXC() __irq
{
                unsigned char temp;
                temp = U0RBR;
          usart0_char(temp);
                U0IER = 0x0001;
                VICVectAddr = 0;
}
void usart1_char(unsigned char temp)
{
                U1THR = temp;
                while(!(U1LSR & 0X20));
                delay(15);
}

void UART1_RXC() __irq
{
    unsigned char temp;
                temp = U1RBR;
          usart1_char(temp);
          U1IER = 0x0001;
                VICVectAddr = 0;
}

void usart0_init()
{
        unsigned int Baud16;
        PINSEL0 = 0x00000005;                                   // selecting P0.0 & P0.1 for usart0 function
        PINSEL0 |= 0X0050000;
        U0FCR = 7;
        U1FCR = 7;
        U0LCR = 0X83;                                                   //DLAB = 1, stopbits =1, data length = 8bit
        U1LCR = 0X83;                                                   //DLAB = 1, stopbits =1, data length = 8bit
        Baud16 = (Fpclk / 16) / UART_BPS;
  U0DLM = Baud16 / 256;
  U0DLL = Baud16 % 256;
        U0LCR = 0X03;                                                   // clearing DLAB bit
  U1DLM = Baud16 / 256;
  U1DLL = Baud16 % 256;
        U1LCR = 0X03;                                                   // clearing DLAB bit



        VICIntSelect=0x00000000;

        U0IER = 0x0001;
        VICIntEnable |= 1 << 6;
        VICVectCntl0= (1<<5)|6;
        VICVectAddr0=(unsigned long) UART0_RXC;

U1IER             |= 1;                //-- Enable UART1 peripheral interrupt
VICIntEnable |=  1<<7;      // UART1 interrupt enabled
VICVectCntl1= (1<<5)|7;
VICVectAddr1  =  (unsigned long)UART1_RXC;  // address of the ISR

}

int main()
{
        usart0_init();
        usart0_char('A');
        usart0_char('B');
        usart0_char('D');
        usart0_char('F');
        usart1_char('A');
        usart1_char('B');
        usart1_char('D');
        usart1_char('F');
}