MN 137 is interesting because it looks like it can shed some light on what vitakka & vicāra mean.
vicara (note short ‘a’) seems to be used synonymously with vicāra, in the context below. I’ve added hyperlink sectioned booksmarks to MN 137 so you can easily see what’s going on and get a sense of the larger picture.
especially when you compare the section (5. 18 mental neighborhoods) with
(gradual replacement of pleasures…), it’s clear the latter section is talking about gradually moving through the first 8 samadhi attainments (4 jhanas, 4 arupa attainments). Then the 18 mental neighborhoods would be referring to what’s possible for vicara and vicāra to do in the rupa first jhāna. Of the 3 translations shown below, I like Bodhi’s the best, it’s the most literal and straightforward (18 mano-upa-vicarati = 18 mental explorations). Bhante @sujato, why “neighborhood”? I’ve been searching for “vicara” (not vicāra) in all the suttas, it doesn’t seem to occur very often. In KN Snp, the context definitely seems to be “wandering and exploring”.
KN Pe commentary for first jhana seems to equate vicara and vicāra as well.
vicarati: goes about; wanders. (vi + car + a)
Vicarati [vi+carati] to go or move about in (loc.), to walk (a road=acc.), to wander Sn 444 (raṭṭhā raṭṭhaŋ vicarissaŋ, fut.), 696 (dhamma – maggaŋ); Nd1 201, 263 Pv iii.73 (aor. vicari); DhA i.66; PvA 4, 22, 33, 69 120, 185 (=āhiṇḍati); Sdhp 133. – In Sn often with loke (in this world), e. g. Sn 466, 501, 845, 846, 864. ↔ Caus. vicāreti; pp. vicarita, vicārita & viciṇṇa. Cp anu˚.;
MN 137
mano-'pa-vicārā
Upavicāra [upa + vicāra; cp. BSk. upavicāra Divy 19, trsld on p. 704 in Notes by “perplexed by doubts” (?) applying (one’s mind) to, discrimination D iii.245 (domanass˚); M iii.239; S iv.232 (somanass˚ etc.); A iii.363 sq.; v.134; Ps i.17; Dh 8, 85, 284; Vbh 381.
bodhi: mental exploration
“‘The eighteen kinds of mental exploration should be understood. ’1235 “” So it was said. And with reference to what was this said?
“On seeing a form with the eye, one explores a form productive of joy, one explores a form productive of grief, one explores a form productive of equanimity.1236 “” On hearing a sound with the ear…On smelling an odour with the nose…On tasting a flavour with the tongue…[217] On touching a tangible with the body… On cognizing a mind-object with the mind, one explores a mind-object productive of joy, one explores a mind-object productive of grief, one explores a mind-object productive of equanimity. Thus there are six kinds of exploration with joy, six kinds of exploration with grief, and six kinds of exploration with equanimity. So it was with reference to this that it was said: ‘The eighteen kinds of mental exploration should be understood.’
sujato: mental neighborhoods
Aṭṭhārasa manopavicārā veditabbā’ti—iti kho panetaṃ vuttaṃ. Kiñcetaṃ paṭicca vuttaṃ? ‘Cakkhunā rūpaṃ disvā somanassaṭṭhānīyaṃ rūpaṃ upavicarati, domanassaṭṭhānīyaṃ rūpaṃ upavicarati, upekkhāṭṭhānīyaṃ rūpaṃ upavicarati. Sotena saddaṃ sutvā … pe … ghānena gandhaṃ ghāyitvā … jivhāya rasaṃ sāyitvā … kāyena phoṭṭhabbaṃ phusitvā … manasā dhammaṃ viññāya somanassaṭṭhānīyaṃ dhammaṃ upavicarati, domanassaṭṭhānīyaṃ dhammaṃ upavicarati, upekkhāṭṭhānīyaṃ dhammaṃ upavicarati. Iti cha somanassūpavicārā, cha domanassūpavicārā, cha upekkhūpavicārā, aṭṭhārasa manopavicārā veditabbā’ti—iti yaṃ taṃ vuttaṃ idametaṃ paṭicca vuttaṃ. (5)
‘The eighteen mental neighborhoods should be understood.’ That’s what I said, but why did I say it? Seeing a sight with the eye, you linger in the neighborhood of a sight that’s a basis for happiness or sadness or equanimity. Hearing a sound with the ear … Smelling an odor with the nose … Tasting a flavor with the tongue … Feeling a touch with the body … Becoming conscious of a thought with the mind, you linger in the neighborhood of a phenomena that’s a basis for happiness or sadness or equanimity. So there are six neighborhoods near happiness, six neighborhoods near sadness, and six neighborhoods near equanimity. ‘The eighteen mental neighborhoods should be understood.’ That’s what I said, and this is why I said it.
thanissaro: explorations
“‘The eighteen explorations for the intellect should be known’: Thus was it said. And in reference to what was it said? Seeing a form via the eye, one explores a form that can act as the basis for happiness, one explores a form that can act as the basis for unhappiness, one explores a form that can act as the basis for equanimity. Hearing a sound via the ear… Smelling an aroma via the nose… Tasting a flavor via the tongue… Touching a tactile sensation via the body… Cognizing an idea via the intellect, one explores an idea that can act as the basis for happiness, one explores an idea that can act as the basis for unhappiness, one explores an idea that can act as the basis for equanimity. The eighteen explorations for the intellect should be known’: Thus was it said. And in reference to this was it said.
Cakkhu. 
