PLT OF C172 DESCRIBES NMAC WITH ANOTHER ACFT OVER IGN VOR WHILE ON VFR ARR TO POU.

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I WAS APCHING POU FOR A FULL-STOP LNDG. I CONTACTED POU TWR WHEN I WAS MORE THAN 10 NM AWAY AND WAS TOLD TO RPT WHEN I WAS OVER THE IGN VOR FOR A STRAIGHT-IN APCH TO RWY 24. SINCE I WAS HEADING DIRECTLY FOR POU AT THAT POINT; I TURNED SLIGHTLY N; TO CROSS OVER THE VOR. JUST AFTER I CALLED THE TWR TO RPT THAT I WAS OVER THE VOR; ANOTHER ACFT RPTED THAT IT WAS APCHING THE VOR (FROM THE NE) AND COULD SEE A CESSNA (ME) ABOUT 50 FT ABOVE IT. I QUICKLY LOOKED TO MY R; SAW THE OTHER ACFT; AND BEGAN TO CLB. THE TWR THEN ASKED ME IF I COULD SEE THE OTHER ACFT; AND I RESPONDED THAT I COULD AND WAS ALREADY CLBING. HE ASKED ME IF I COULD CIRCLE TO THE R AND COME BACK OVER THE VOR FOR MY LNDG APCH; AND I RESPONDED IN THE AFFIRMATIVE. THE OTHER ACFT CONTINUED TOWARDS THE ARPT AND THEN DECLARED A MISSED APCH. APPARENTLY IT WAS PERFORMING A PRACTICE INST APCH. AS IT WAS CLBING AWAY FROM THE ARPT; THE TWR CTLR SPOKE VERY SHARPLY TO THE PLT AND INFORMED HIM THAT WHAT HE HAD JUST DONE WAS VERY DANGEROUS; AND HE SHOULD NOT DO IT AGAIN. I BELIEVE THAT WHEN THE OTHER ACFT FIRST CONTACTED THE TWR; IT MAY HAVE ALREADY BEEN INSIDE THE CLASS D AIRSPACE. THE VOR IS ONLY 4 NM FROM THE ARPT; AND HE WAS NEARLY ON TOP OF IT WHEN HE RPTED SEEING ME THERE. IN HIS ADMONISHMENT TO THE PLT; THE TWR CTLR TOLD HIM THAT HE NEEDED TO TAKE CARE IN THE FUTURE TO BE SURE THAT HE WAS IN CONTACT WITH THE TWR WELL BEFORE ENTERING ITS AIRSPACE. I DON'T BELIEVE WE WERE REALLY ONLY 50 FT APART. I HAD NOT YET DSNDED TO PATTERN ALT (1200 FT) AND WAS STILL AT ABOUT 1800 FT WHEN I CROSSED THE VOR. ASSUMING THAT THE OTHER ACFT WAS CLOSER TO 1200 FT THAN I WAS; WE WERE PROBABLY AT LEAST A COUPLE HUNDRED FEET APART. I KNOW THAT WHEN I SAW HIM; HE DIDN'T SEEM SO CLOSE AS TO CAUSE ME SERIOUS ALARM; AND I CLBED ONLY TO INCREASE THE SEPARATION A BIT MORE. I KNOW I DIDN'T DO ANYTHING WRONG; BUT I WAS DISAPPOINTED IN MYSELF FOR NOT HAVING SEEN THE OTHER ACFT SOONER. I AM A NEWLY MINTED PVT PLT; HAVING EARNED MY CERTIFICATE ONLY 3 WKS AGO; AND THIS WAS MY FIRST UNSUPERVISED SOLO XCOUNTRY FLT TO AN ARPT I HAD NEVER VISITED BEFORE. WHILE THE VISIBILITY WAS RPTED AS 10 MI; AND THAT SEEMED FAIRLY ACCURATE; IT WAS QUITE HAZY; AND OBJECTS AT A DISTANCE WERE NOT VERY DISTINCT. SO AS I WAS APCHING THE VOR I WAS CONCENTRATING ON FINDING IT ON THE GND AND MAKING SURE I COULD SEE THE ARPT'S RWY TO MY L. I WAS NOT DOING A GOOD JOB OF SCANNING FOR TFC IN OTHER DIRECTIONS. THIS INCIDENT HAS SERVED TO REINFORCE THE FOLLOWING POINTS THAT I LEARNED DURING MY TRAINING; AND WILL REMIND ME TO FOLLOW THEM MORE THOROUGHLY IN THE FUTURE: EVEN THOUGH I AM UNDER POSITIVE CTL OF AN ARPT'S TWR; I HAVE A RESPONSIBILITY TO KEEP DILIGENTLY SCANNING FOR TFC IN ALL DIRECTIONS. SOME OF THE MOST DANGEROUS AIRSPACE AROUND IS IN THE VICINITY OF A VOR; SINCE ACFT ARE CONVERGING ON IT. AIRSPACE IN THE VICINITY OF AN ARPT IS ALSO QUITE DANGEROUS SINCE ACFT CAN BE ALMOST ANYWHERE. ACFT ON AN INST APCH ARE LIKELY TO BE ON THE EXTENDED CTRLINE OF THE ACTIVE RWY; SO THIS IS AN AREA WHERE SCANNING FOR TFC IS EVEN MORE CRITICAL.