I have some old tablets what say Sodium Dichloroisocyanurate and 'Sterilising Tablets' on them and am wondering if these when diluted will be suitable to clean vessel/equipment before brewing?
Without knowing what the tablets were intended for I wouldn't use them.
While in high concentrations a Sodium Dichloroisocyanurate solution would work for sanitizing but would be a poor choice for brewing because of the residual chlorine odor. You really want your final wash / rinse to be with a no rinse sanitizer.
Sodium Dichloroisocyanurate tablets could be very weak doses intended to sanitize drinking water. Which would be a poor sanitizer for cleaning equipment, as it's only strong enough to make water safe to drink.