This situation occurs with vendors in the Pale Heart and Neomuna. The commonality between these two destinations is that even after unlocking all weapon patterns, certain activities (such as Overthrows and Terminal Overload) are still required to obtain random rewards, during which destination Engrams are generated.
Once we have unlocked all the patterns for the destination weapons, there seems to be no worthwhile option to utilize these Engrams. It is hoped that more purchasing options can be provided for these vendors.
Pale Heart: Exotic class items (41 Engrams for one, similar to Xur) and Ergo Sum.
Neomuna: Focused Decoding options for Terminal Overload rewards—Volta Bracket, Circular Logic, and Synchronic Roulette.
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'I got 99 engrams, and the roll I want ain't one' TBH, these engrams, and those for Xur, Rahool and The Helm are all just glimmer/enhancement core fodder for me, and have been for some time