Using a mod that remaps vanilla creatures to rebalance them like I do? Then you may run into the problem that both Eocarcharia and Vectispinus rely on vanilla spawns. Luckily, I have come up with a solution! Plop the following lines of code into your game.ini and Vectispinus should spawn on its own (you may need to tweak them a little to prevent over-spawning).

Regular Vecti + Alpha:
ConfigAddNPCSpawnEntriesContainer=(NPCSpawnEntriesContainerClassString="DinoSpawnEntries_InlandWater_C",NPCSpawnEntries=((AnEntryName="Vectispinus",EntryWeight=0.027,NPCsToSpawnStrings=("Vectispinus_Character_BP_C"),((AnEntryName="AlphaVectispinus",EntryWeight=0.0055,NPCsToSpawnStrings=("Vectispinus_Alpha_Character_BP_C"))),NPCSpawnLimits=((NPCClassString="Vectispinus_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.007),((NPCClassString="Vectispinus_Alpha_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.00035)))
ConfigAddNPCSpawnEntriesContainer=(NPCSpawnEntriesContainerClassString="DinoSpawnEntries_Ragnarok_CenterWater_C",NPCSpawnEntries=((AnEntryName="RagnarokVectispinus",EntryWeight=0.027,NPCsToSpawnStrings=("Vectispinus_Character_BP_C"),((AnEntryName="RagnarokAlphaVectispinus",EntryWeight=0.0055,NPCsToSpawnStrings=("Vectispinus_Alpha_Character_BP_C"))),NPCSpawnLimits=((NPCClassString="Vectispinus_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.007),((NPCClassString="Vectispinus_Alpha_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.00035)))
ConfigAddNPCSpawnEntriesContainer=(NPCSpawnEntriesContainerClassString="DinoSpawnEntries_spinosaurus_C",NPCSpawnEntries=((AnEntryName="LostIslandVectispinus",EntryWeight=0.027,NPCsToSpawnStrings=("Vectispinus_Character_BP_C"),((AnEntryName="LostIslandAlphaVectispinus",EntryWeight=0.0055,NPCsToSpawnStrings=("Vectispinus_Alpha_Character_BP_C"))),NPCSpawnLimits=((NPCClassString="Vectispinus_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.007),((NPCClassString="Vectispinus_Alpha_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.00035)))

X-Vecti:
ConfigAddNPCSpawnEntriesContainer=(NPCSpawnEntriesContainerClassString="Gen1_Bog_Spino_DinoSpawnEntries_C",NPCSpawnEntries=((AnEntryName="BogVectispinus",EntryWeight=0.15,NPCsToSpawnStrings=("Vectispinus_x_Character_BP_C"))),NPCSpawnLimits=((NPCClassString="Vectispinus_x_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.15)))

R-Vecti:
ConfigAddNPCSpawnEntriesContainer=(NPCSpawnEntriesContainerClassString="DinoSpawnEntriesGardens_RiversLand_Gen2_C",NPCSpawnEntries=((AnEntryName="RockwellVectispinus",EntryWeight=0.014,NPCsToSpawnStrings=("Vectispinus_r_Character_BP_C"))),NPCSpawnLimits=((NPCClassString="Vectispinus_r_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.0004)))

Unfortunately, Eocarch has no blueprint shortcut, so I don't know how to implement .ini spawners for it.