The ECU, key and immobiliser are all coded together so you can't just replace one part of the system or bypass the immobiliser.
Either refit the immobiliser and original ecu and get a new key coded to these or you will need to fit a new matching key, immobiliser and ECU from vitara of the same era (and same ECU part number)
You could fit a ecu from an earlier model without an ECU but there would be some rewiring involved that isn't very simple unless you are familiar with automotive electrics, Not something the majority of car garages would be willing to tackle as its a very time consuming job.
https://www.ebay.co.uk/itm/274858351195 ... SwY6Rg5Gro this guy is breaking a 4u, could message to see if he has the key, ignition barrel, ecu, imobiliser and lock set still?