HILL 881, VIETNAM - (UPI) - They  sleep by  day and  fight by night.  They read Bibles and scratch "God Help Me" on their helmets.

For  entertainment  they  watch U.S. jets  bombing  the communist  divisions surrounding  them or  try to  kill  rats scrambling  about  their  trenches  and bunkers.  The Marines of  this Khe Sanh outpost have their own  philosophy: "There's  two  ways to get off Hill 881 - flown  off  or  blown  off,"  said  Sgt. Joseph Michael Jones of Chatahoochee, Fla.

Since  the  communists  began shelling  Khe  Sanh  and  this  hilltop  Jan. 21, 21 Marines have  died on  881.  Another 140 have  been wounded.  Fog  is an enemy too.  Three  wounded Leathernecks died on 881 for want of a helicop- ter that couldn't get here. The wreckage of one lies down the slope.  Darkness is an ally.  It  hides the men of 881 from the North Vietnamese observers and gunners  lurking  unseen  on  the  twin  mount called Hill 881-North, just 500 yards away.

The  Marines captured Hill 881 last May in one of their fiercest battles.  They have held it ever since.  But no one kids himself.  "If they really want to take this hill, they can take it," said Capt. William Dabney of Saluda, Va., son-in- law of retired Gen. "Chesty" Puller, America's most decorated Marine.  "We can  hold against a  regiment.  They would  take  tremendous  casualties," he said.  At  night,  Dabney's  men  wiggle  out  of  the  of  the bunkers and into trenches. They crawl out to pick up the supplies the helicopters have dropped and to fire their mortars and howitzers.  In Darkness  they make more  sand- bags.

There's a hellish little ceremony at dawn that makes them proud, up  here on 881.  Three  Marines  race at first light from a bunker with an American flag. Two of them hoist it to the top of a 15-foot radio antenna.  The third blows a bugle, "Colors."  It is a rusty rendition.  But it  is a symbol of life  atop  881. At nightfall  the Marines retire that day's flag, to send the family of a Marine slain on  the hill.  They have  a stockpile of flags.  Cpl.  Marvin C. Brown  of Abilene, Tex., Lance Cpl. Barry L. Ulrich of Reinholds, Pa., and Lt. Owen S. Matthews of Richmond, Va., raised  the first of  these flags Feb. 17.   It is  like an old tradition now.

Lt. Thomas Biondo  of Albany, N.Y.,  has been on Hill 881 for 200 days.  He takes  an  occasional  break  for  "rest and recuperation"  at Khe Sanh,  that cockpit combat. Some Marines on the hill have had to postpone their regular R&R  trips  to  places like Hong Kong because  the choppers couldn't  come. Sometimes  they went without food, once for three days;  there was too much communist fire for the choppers.

"Stay  down  and  stick  around." is  the byword here.  Stay in  your  bunker during  the day and you  live.  Daylight is  the time for quiet talks,  the  Bible reading.   A  few  have daytime  jobs,  like  "The  mightiest  Corporal  in  the world."  That's  what  they  call Cpl. Robert J. Arrotta of La Canada, Calif., because he goes out in  the midday sun  to coordinate the air  strikes  around the hill.

Capt. John T. Esslinger of Ephrata, Pa.,  makes his home in a bunker dug by former  communist  occupants.   Housekeeping  is a bother, what with the in- coming shells occasionally unearthing more communist bodies left over from May's fighting.

A  medic, Navy  Corpsman  James R. Mathis  of  Houston,  concentrates  on rats. He woke one day to see a rat on his chest.  The rat stared at him.  A few days later another rat wiggled into Mathis' hair.  "Enough," said Mathis.  He fashioned  a  blowgun.   In  the  darkness of his bunker  he waits for a sound. Then  he turns on his flashlight and uses  the blowgun.   So far,  he has killed five rats plus one "probable" -  a rat that fled with a dart in its side.