When you say stock VSTi, that can only mean an EXS 24 instrument.
Which one have you loaded? Do you see instrument names like these if you open the Library while the EXS Cello track is selected? (see attachment)
The way I like to work is to select the instrument named "Cellos" vs the other more specifically named instruments. Reason being that it contains all of the the listed articulations.
Articulations are changed via Modulation (continuous controller #1) data.
Copy and paste the following into the Media>Notes for the Cello track as a quick visual reference. Draw in or record CC#1 data a needed.
VC-L_Legato_f CC#1 0-9 vel 111-127
VC-L_Legato_mf CC#1 0-9 vel 71-110
VC-L_Legato_p CC#1 0-9 vel 0-70
VC_stac_downbow CC#1 10-36 vel 0-127
VC_stac_upbow CC#1 10-36 vel 0-127
VC-L_tremolo_mf CC#1 37-63 vel 0-127
VC_trills_half tone CC#1 64-90 vel 0-127
VC_trills_whole tone CC#1 91-117 vel 0-127
VC_pizz_f CC#1 118-127 vel 111-127
VC_pizz_p CC#1 118-127 vel 0-110
FYI, The Logic parlance is software instrument vs VSTi.
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