Thats not the "common way" because i use the vanilla launcher for creating the championships. But while you are asking - thats the way how it´s working at least for me:
AI strenghts is set for all cars, according to your level.
But creating a season with the vanilla launcher i have always to replace the vanilla drivers with the real names first. And here could be the trick - you have to replace strong vanillas drivers with strong real ones and obviosly the avarage or the week ones with there real counterparts.
{"name":"Roberto Moreno","nation":"BRA","car":"asr_1991_benetton_b191","skin":"19_moreno_r8","ballast":30,"restrictor":0},{"name":"Nelson Piquet","nation":"BRA","car":"asr_1991_benetton_b191","skin":"20_piquet_r8","ballast":20,"restrictor":0},{"name":"Martin Brundle","nation":"GBR","car":"asr_1991_brabham_bt60y","skin":"07_brundle_r8","ballast":40,"restrictor":0},{"name":"Mark Blundell","nation":"GBR","car":"asr_1991_brabham_bt60y","skin":"08_blundell_r8","ballast":45,"restrictor":0},{"name":"Emanuele Pirro","nation":"ITA","car":"asr_1991_dallara_191","skin":"21_pirro_r8","ballast":45,"restrictor":0},{"name":"J.J. Lehto","nation":"FIN","car":"asr_1991_dallara_191","skin":"22_lehto_r8","ballast":35,"restrictor":0},
{"name":"Jean Alesi","nation":"FRA","car":"asr_1991_ferrari_643","skin":"28_alesi_r8_r9","ballast":20,"restrictor":0},{"name":"Bertrand Gachot","nation":"BEL","car":"asr_1991_jordan_191","skin":"32_gachot_r8","ballast":35,"restrictor":0},{"name":"Andrea de Cesaris","nation":"ITA","car":"asr_1991_jordan_191","skin":"33_decesaris_r8","ballast":30,"restrictor":0},{"name":"Thierry Boutsen","nation":"BEL","car":"asr_1991_ligier_js35b","skin":"25_boutsen_r8_r9","ballast":50,"restrictor":0},{"name":"Ayrton Senna","nation":"BRA","car":"asr_1991_mclaren_mp4-6","skin":"1_senna_r8","ballast":0,"restrictor":0},{"name":"Gerhard Berger","nation":"AUT","car":"asr_1991_mclaren_mp4-6","skin":"2_berger_r8","ballast":10,"restrictor":0},{"name":"Pierluigi Martini","nation":"ITA","car":"asr_1991_minardi_m191","skin":"23_martini_r8","ballast":35,"restrictor":0},{"name":"Satoru Nakajima","nation":"JPN","car":"asr_1991_tyrrell_020","skin":"3_snakajima_r8_r9","ballast":40,"restrictor":0},{"name":"Stefano Modena","nation":"ITA","car":"asr_1991_tyrrell_020","skin":"4_smodena_r8_r9","ballast":25,"restrictor":0},{"name":"Nigel Mansell","nation":"GBR","car":"asr_1991_williams_fw14","skin":"5_mansell_r8_r9","ballast":5,"restrictor":0},{"name":"Ricardo Patrese","nation":"ITA","car":"asr_1991_williams_fw14","skin":"6_patrese_r8_r9","ballast":10,"restrictor":0},{"name":"Ivan Capelli","nation":"ITA","car":"asr_1991_leytonhouse_cg911","skin":"16_capelli_r7_r8_r9","ballast":45,"restrictor":2},{"name":"Alain Prost","nation":"FRA","car":"asr_1991_ferrari_643","skin":"27_prost_r8_r9","ballast":15,"restrictor":0},{"name":"Eric Bernard","nation":"FRA","car":"asr_1991_larrousse_lc91","skin":"29_bernard_r8","ballast":45,"restrictor":2},{"name":"Aguri Suzuki","nation":"JPN","car":"asr_1991_larrousse_lc91","skin":"30_suzuki_r8","ballast":45,"restrictor":2},{"name":"Mikka Häkkinen","nation":"FIN","car":"asr_1991_lotus_102b","skin":"11_hakkinen_r8","ballast":40,"restrictor":2},{"name":"Gianni Morbidelli","nation":"ITA","car":"asr_1991_minardi_m191","skin":"24_morbidelli_r8","ballast":45,"restrictor":0},{"name":"Maurizio Gugelmin","nation":"BRA","car":"asr_1991_leytonhouse_cg911","skin":"15_gugelmin_r7_r8_r9","ballast":50,"restrictor":2},
{"name":"Michele Alboreto","nation":"ITA","car":"asr_1991_footwork_fa12","skin":"10_johansson_r7_r8","ballast":50,"restrictor":2}
I am also using the 1992 tracks with the 1991 ASR cars, so if you want you could try my chamionship.file preset. I can upload it if you want. Shoud work