You can still find the original Japanese freeware version (v1.08) on legacy community sites and the developer's blog . While free, it may require tools like AppLocale to run on modern non-Japanese Windows systems.
The Steam version offers several DLCs that add characters, story modes, and technical features: Vanguard Princess Free Download (v1.8.7 & ALL DLC)
The version published by eigoMANGA on Steam is a localized PC port that includes bug fixes and various paid DLC expansions. You can still find the original Japanese freeware
An updated, uncensored version featuring the "Daughter of the Devil," new stages, and alternate endings. " new stages
Adds the final boss as a playable character in Arcade and Two-Player modes.
Often available as a free update/DLC on Steam, adding uncensored content and minor gameplay adjustments. Vanguard Princess - Steam DLC Page