If you write an MVC statement with an equated value as the sending OPERAND, then the assembler will try to RESOLVE that operand value as a base and displacement, it will not necessarily throw an error at assembly, but the results at execution will be unpredictable and may well give rise to a protection exception. 

The point of the MVI statement is that the SINGLE byte sending operand value is assembled as part of the instruction itself and does not have to be 'fetched' at execution time, therefore if you are only MOVING a single byte of fixed value, then an MVI will be marginally more efficient than an MVC

If you write an MVC statement with an equated value as the sending operand, then the assembler will try to resolve that operand value as a base and displacement, it will not necessarily throw an error at assembly, but the results at execution will be unpredictable and may well give rise to a protection exception. 

The point of the MVI statement is that the single byte sending operand value is assembled as part of the instruction itself and does not have to be 'fetched' at execution time, therefore if you are only moving a single byte of fixed value, then an MVI will be marginally more efficient than an MVC