there could be 2 reasons for this kidnap.
1] vidith might want to hide his criminal past from family.some how rishi comes to know about this.so to keep it secret --vidith kidnapped rishi.
2] may be vidiths opponents might have kidnapped rishi to teach lessons to vidith.
his opponents want some secret from vidith [1st time when vidith talked on phone some angry exchanges taken place.] which vidith not giving.so to blackmail him --his opponents might have kidnapped rishi.
i think 2nd option is more likely if cvs don't want to spoil indira --vidith relation in future.
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p.s--this post is based on what i read in forum posts.as i skipped last 5 episodes.so it is purely my assumption as i don't know --actually what happened in last 5 days.