Since BF3 is deficient two electrons from the octet, a characteristic reaction is to react with lone pair donors. Acceptance of a pair of electrons makes BF3 a Lewis acid. The electron pair donor NH3 is a Lewis base. The product of the reaction between NH3 and BF3 is sometimes written H3N→BF3. There are two atoms of interest, N and B. As in any other molecule containing more than one 'central' atom, it is necessary to execute a calculation for each centre.
The geometry of H3N→BF3. You can use your mouse to manipulate the molecule in the right hand "Jmol" image.
An alternative representation (below) of BF3NH3 places a positive charge on nitrogen and a negative charge on fluorine. The octet rule is still satisfied for each atom and it is now not necessary to write in an arrow to distinguish the B-N bond from other single bonds. In some ways this is an advantage since it removes the impression that a dative bond is different from other bonds. The VSEPR calculation works perfectly well on this representation.