If you are planning to buy clothes in Vaci street from international brands stores, then definitely, Vaci street is the best option.
The reason is that international brands stores prices are consistent throughout Budapest and you can do shopping while exploring Budapest.
Hence, Vaci street is not expansive for international brand shopping.
However,if you plan to various souvenirs shop Budapest at Vaci Street, you may find some expansive ones.Therefore, you need to do more exploration to find the best price while shopping at Vaci Street.