Did you make the necessary ini tweaks for Darn?
If not, copy the following into \My Games\FalloutNV\Fallout.ini, in place of the existing [Fonts] section:
[Fonts]
;sFontFile_1=Textures\Fonts\Glow_Monofonto_Large.fnt
sFontFile_1=Textures\Fonts\DarN_FranKleinBold_14.fnt
;sFontFile_2=Textures\Fonts\Monofonto_Large.fnt
sFontFile_2=Textures\Fonts\DarN_FranKleinBold_16.fnt
sFontFile_3=Textures\Fonts\Glow_Monofonto_Medium.fnt
;sFontFile_4=Textures\Fonts\Monofonto_VeryLarge02_Dialogs2.fnt
sFontFile_4=Textures\Fonts\DarN_Sui_Generis_Otl_10.fnt
sFontFile_5=Textures\Fonts\Fixedsys_Comp_uniform_width.fnt
;sFontFile_6=Textures\Fonts\Glow_Monofonto_VL_dialogs.fnt
sFontFile_6=Textures\Fonts\DarN_Sui_Generis_Otl_13.fnt
;sFontFile_7=Textures\Fonts\Baked-in_Monofonto_Large.fnt
sFontFile_7=Textures\Fonts\DarN_Libel_Suit_Otl_24.fnt
sFontFile_8=Textures\Fonts\Glow_Futura_Caps_Large.fnt
sFontFile_9=Textures\Fonts\NVFont_Test.fnt