"Herstellern und Wiederverkäufern/Händlern installiert/zu installieren.

"multimodal" durch viele Google-Hits belegt.
"proprietary" laut Ernst: "firmeneigen", "Marken-" "Hersteller-"
"vendor" ist der "Wiederverkäufer", (Zwischen) "Händler".
Ich habe den Satz ein bisschen umgestellt, da Hersteller und Händler sinngemäß hier zusammen gehören. Persönlich ziehe ich im Deutschen immer verbale Konstruktionen vor, wenn sie nicht zu lang werden, da Nominalbildungen immer etwas gestelzt klingen.

I presume it is about some company providing system integration or some kind of custom solutions including both software and hardware. Hence:

1) 'non-proprietary' = their solutions don't include e.g. proprietary software of their own development - that is, the solutions are built on standard hardware/software components (meaning: no hidden costs like ongoing license fee payments);

2) 'multi-modal' = able to operate in various modes (meaning: excellent flexibility/adaptability to the client's needs, also easy adapting to new tasks/requirements, once installed);

3) 'vendor-neutral' = the components are standard, so you can buy them from any vendor (meaning: the client is not obliged to buy any components through that company, therefore can freely choose the vendor basing on pricing or other considerations)

Not 100% sure about item 2); no doubt about 1) and 3), though.

Hope this helps.