Vynthra benefits from Reliable and Affordable Ranged DPS (Cha'Kru Chas'ki, Endari Needler, Marran Sharpshooter)
Her Roster already provides excellent and affordable melee units and support casters
To complement Vynthra's functionality, I recommend your equipped items to be:
Cha'Kru Temple of Anqas (Cha'Kru Human Units)
The Cha'Kru Chas'ki is the Best DPS Merc, very affordable, good health, good DPS
Cha'Kru units are faster than your regular roster, this can be used as an advantage, speeding up the early game ramp-up
Cha'Kru Anqas units are quite affordable and don't suffer from gimmicks
Rageborn is not worth getting; Cultists can be useful in small quantities to aid Weavers
Gensis Core (+1hp/s Regeneration outside of Combat per level)
This item has low utility until level 3 sadly
Not very needed, given Vynthra's plethora of healing abilities, but there aren't a lot of other picks 2.5 Elderbastion Shell (+15% hp to all buildings per level)
Much more useful than the Gensis Core, if you bother with static defenses
Vynthra does have the best static defenses, but I personally found they are durable enough as is
Enigmatic Capacitor (5% Faster Ability Cooldowns per Level)
Affects Vynthra herself, Harbinger, Mandragora and Lasher abilities
Vynthra only has one ability that benefits from the item
However, a 25% cooldown reduction to said ability can greatly increase the rate at which Faction Power is gained
The Frenzy ability that all of Vynthra's units have is also affected
Utility depends on build composition and Micro abilities
Vynthra's Frenzy and Infusion abilities grant significantly more mileage to all Merc Factions
However, Cha'Kru Anqas remains the most cost-effective in terms of Ranged DPS
250 Hexite for 3 Chas'Ki (11dps each; +50% vs Medium)
100 Hexite, 200 Flux for 3 Marran Sharpshooters (15dps each)
75 Hexite, 200 Flux for 2 Corsair Deadeyes (14dps each; +75% vs Light)